Infrastructure / Migration / From One Instance to Another
The two containers do not merge. You pour one into the other.
And throughout all this time, the two companies must continue to work together.A merger, an acquisition, a business spin-off, or a group bringing its subsidiaries back under one roof: in all four cases, two environments Microsoft 365 must become one. The technical aspects can be planned. What determines success is the period in between —the time when your people are still on both sides and still need to see each other, communicate, and meet.
Three questions will determine the schedule
These questions should be asked before even looking at any tools. None of them are technical; they focus on what your organization can handle and on the timeline the project requires.
How long should the two sides continue to talk to each other?
A few days, if the acquired company is small and everything wraps up over the same weekend. Several months, if the two teams start working together as soon as the deal is signed. This timeframe determines everything else: the longer it is, the more effort is needed to truly build a sense of unity.
We estimate the duration before choosing the method.
What Needs to Go, and What Needs to Stay
In a merger, almost everything is included. In a spin-off, on the other hand, you have to separate out one division without taking the rest with it —and then prove that the rest is no longer accessible. These are two different projects, and the second is the more difficult of the two.
It takes longer to separate than to bring together.
When a domain name changes registrants
It’s the only date we can’t freely change, and the only constraint of the project that can’t be worked around. It sets the transition weekend, and the entire plan is built backward from there.
The date is what matters, not the other way around.
A domain name can only exist in one location at a time.
This is a Microsoft policy, not a matter of choice. As long as societe.ch is verified in the source tenant, it cannot be verified in the destination tenant. Removing it from one side and adding it to the other takes a short but tangible amount of time, and during that time , email for that domain does not work anywhere.
Practical implication: The email system switchover happens in an instant—it’s not a gradual transition. It takes place on a Saturday, is prepared for over the course of several weeks, and is rehearsed beforehand. Anything that can be migrated beforehand is migrated; anything that can’t must wait until that moment.
What works between the two sides before the switch occurs
This is the part that plans migration overlook, and it’s the part your users experience every day for weeks on end. Some things can be built, others cannot —it’s better to know that before making any promises.
| What Your People Want to Do | Is this possible before the switchover? | What You Need to Do to Get It |
|---|---|---|
| Writing from one side to the other | Yes, right from the first day. | Nothing out of the ordinary: both areas exist, and mail is delivered just as it is with any other external correspondent. |
| View availability on the calendar | Yes, and that's what makes the biggest difference in everyday life. | A relationship between the two organizations, established by both sides. It is an adjustment, not a new development. |
| Discuss and meet in Teams | Yes, conversations and meetings. | Permission to communicate with the other organization must be enabled in both tenants. Be sure to request this early—it is often disabled by default. |
| Find a coworker in the directory | Partly. The people appear, but it's not the same directory. | Synchronize the address book between both sides, if time permits. Otherwise, skip it. |
| Working on the Same Files | Yes, by personal invitation. | Opening the external share to the other domain, and nothing beyond that. |
| View conversation history | No. It stays on the side where it was written. | No setting can achieve this. This is a limitation that should be communicated to the teams beforehand, not discovered afterward. |
These mechanisms are documented by Microsoft, and we double-check them before each proposal: the settings for cross-organizational collaboration are the ones that change most frequently in Microsoft 365.
What Can Be Migrated, and What Cannot
The second column is the one that counts. Anything not listed there must be re-entered by hand in the destination file, and that work is billed just like the migration form itself.
| What your people have | What's Happening on the Other Side | What Still Needs to Be Done |
|---|---|---|
| Mailboxesand Archives | Everything: messages, folders, calendar, contacts, online archive. | Nothing, as long as the preparation is done. That's the most reliable part of the project. |
| OneDrive | All content, including the directory structure. | Sharing links sent in the past no longer work: they point to the old address. |
| Sites SharePoint | Documents, libraries, lists, and the basics of permissions. | The customizations and automations set up on a website are being reviewed one by one. |
| Teams Teams | Structure and files: channels, documents, members. | The tab layout and the apps added to the channels are being reset. |
| Conversations Teams | Very little. It's not a limitation of the tool; it's the nature of the data. | Let the teams know that they will retain read-only access for the agreed-upon period, and that this access will then be revoked. |
| Accounts and Passwords | Nothing. The accounts are recreated in the destination tenant. | Each person receives a new password and sets up two-step verification again. This is the busiest day for your support team. |
| Automation and ReportsPower Automate, Power BI | Nothing moves exactly as it is. | To be re-entered in the final statement. To be listed early: this is the budget item most often overlooked. |
| Third-party applications connected to the tenant | Nothing. Each one has its own connection. | Each publisher has its own procedure. They are handled one by one, and some require a new contract. |
| Workstations | Nothing happens automatically: a machine linked to a tenant does not switch tenants on its own. | Re-registration of positions. This is where preparing new machines and managing the fleet make all the difference. |
What matters to you is what happens on the other end, not how it was moved. The capacities described are those we achieve in production, not those listed on a product sheet.
The posts are being re-entered: this is a separate project, and it’s being prepared at the same time.
A machine linked to the original owner must be linked to the new one. Depending on your IT infrastructure, this can range from a silent re-registration to a complete reinstallation. This is the job of workstation management, and it has a much greater impact on the workload on D-Day than migration mailbox configuration.
How Does a Mandate Work?
There are five stages, and the fourth is the only one held on a fixed date. The first three are prepared well in advance; that’s what makes the fourth one so short.
We're taking stock of both sides
Accounts, mailboxes and their storage quotas, sites, teams, automations, applications linked to each tenant, and workstations. The inventory of automations and applications is the one we tend to skip—and it’s also the one that causes budgets to spiral out of control.
We'll come away with a list of what's being migrated and what's being rebuilt.
We're setting up coexistence
Calendar availability, communication via Teams, and file sharing between the two domains. This was set up in just a few days and immediately changed the daily routines of both teams—months before the switchover.
That's also what makes the project acceptable internally: people see a benefit before they have to deal with a constraint.
We migrate in waves, ahead of schedule
Data is transferred before the switchover, in groups of people, multiple times if necessary: a complete initial copy, followed by incremental updates that include only the changes made since then. Your users continue to work as usual during this time.
First, a pilot program—chosen in consultation with you—and we'll only move forward after evaluating its results.
We're switching the domain name
On the agreed-upon Saturday, the domain is removed from the original host, set up on the new host, the final synchronization runs, and email begins arriving at the new location. This is the only irreversible step in the project, and it was tested in a dry run beforehand.
We're here throughout the entire time slot, and on Monday morning as well.
We're closing out the previous tenant properly
Items that haven't been migrated are checked; items that must be retained for legal reasons are removed and stored elsewhere; and subscriptions are canceled on the correct date. A tenant that is overlooked continues to be billed and remains an open door.
This step should be planned at the beginning, not at the end.
Other migration projects we are carrying out
The principle is always the same—an inventory, a period when the two worlds coexist, waves, a shift. What changes is what gets lost along the way, and that depends entirely on the starting point.
From Google Workspace to Microsoft 365
Email, calendars, contacts, and the entire Drive, including shared folders. The tricky part is document conversion: a Google file becomes an Office file, and not all formulas, scripts, and links between documents are preserved.
This is the migration where the collection of homemade documents matters most.
From Microsoft 365 to Google Workspace
The reverse route—and we do that, too. Mailboxes, calendars, OneDrive and libraries SharePoint toward Drive.
Being able to do both is what allows us to advise you on your challenges rather than on what we know how to sell. A service provider who can only see things one way will always assume that’s your way.
From an old email service to Microsoft 365
An email service hosted by a provider, a non-Microsoft email server, an older internal platform. The main challenges stem from the format of the archives and the way shared folders were used.
Calendars and contacts almost always require more work than email.
Network shares to SharePoint
The network drive that everyone calls “S:”. The migration The technical part is easy: the real work is deciding what deserves to be mounted and translating folder permissions into site permissions.
Presenting twenty years of shared experiences exactly as they are merely shifts the disorder elsewhere.
From a third-party storage provider to OneDrive and SharePoint
An online file-sharing service in addition to the one already included in your subscriptions. The benefit is both contractual and technical: one less service to pay for, and a single place to apply your rules.
Sharing links sent externally are something that requires careful handling.
From SharePoint on your servers to SharePoint online
Websites, libraries, lists, and validation feeds. The features developed over the years on the old platform are no longer supported: they are either rewritten using current tools or abandoned.
This is a chance to sort out what was actually useful.
A migration judges herself on Monday morning, not Saturday night.
On Saturday, we check to see if the data has come in. On Monday, we discover things that no one had accounted for: the automation system that sent out invoices, the accounting app linked to the old tenant, and the person who can no longer log in because their two-factor authentication was tied to an old phone.
The inventory determines the budget
Mailboxes and files are counted by volume, and that part comes as no surprise to anyone. What is surprising is the number of automations and applications connected to the tenant—which is always higher than the IT department anticipated.
Coexistence helps avoid upsetting the teams
When the two teams are scheduled to meet in the first month, the handoff becomes a mere administrative formality. When they aren’t scheduled to meet, every week of delay results in missed meetings and workarounds that no one will later undo.
The switch is repeated on a test domain
The domain name changes hands only once. This process is first rehearsed on a test domain—following the same procedure and involving the same people—before it is carried out for real.
What You Need to Know Before Committing
Four points we bring up ourselves on the first date. Everyone has their own standards; none of them is a reason to call it off, but none of them can be discovered along the way without a high cost.
Conversation history is not transferred
This is the most difficult limit to deal with, because it affects everyone and is revealed on Mondays. The messages posted in the old forum remain there.
Here's what we do: We maintain read-only access to the old environment for a period agreed upon with you, and we notify the teams beforehand—not afterward. Anything that must be retained for legal reasons is removed and stored separately.
Each person will need to set up two-step verification again
The accounts are new in the destination tenant: the password and two-factor authentication are not carried over. For a company with 200 employees, that means 200 steps to take all on the same morning.
Here's what we do: we prepare a one-page procedure, send it out in advance, and provide extra support during the first two days. This is included in the budget from the start—it isn't added later.
A divestiture takes longer than a merger
Spinning off a division requires deciding, file by file, what goes and what stays—and then proving it. This sorting process is your organization’s responsibility, not that of a service provider.
Here's how it works: We provide the inventory and tools needed to make quick decisions, and we work with your business unit managers to sort through everything. This will take some time on your end, and it's best to know that up front.
Applications connected to the tenant are processed one by one
Every software program connected to your system—accounting, human resources, electronic signatures, backup—has its own procedure, and some vendors charge for the change.
Current practice: We identify them in Phase 01 and open files with the publishers while the data is being migrated. None of these deadlines are on the critical path if the process is started early enough.
Related Topics
A migration component handles the address book, email, and files. Each is handled separately:
Microsoft Entra ID
The incoming directory, synchronization from your servers, and the three authentication methods—the decision that must be made before anything else.
Read the page MessagingExchange, on your servers and online
Your starting point isn't a tenant but a server Exchange hosted on your own server? The migration The Microsoft 365 is set up differently.
Read the page FilesMicrosoft SharePoint
What happens to documents once they've been migrated: how the sites are organized, who has what permissions, and what we decide not to publish.
Read the pageLet's start by taking stock of both sides
One hour with the engineers who will carry out the work, focusing on your two environments as they currently stand: the volumes, the connected applications, and the deadline set for the operation. You’ll leave knowing what needs to be migrated, what needs to be rebuilt, and which Saturday to aim for.
Renens, Sion, Châtel-Saint-Denis. +41 21 806 37 15 — [email protected]