When you buy a company, the IT question that takes longest to answer is what happens to their Microsoft 365 tenant. Here are the questions owners and controllers ask us in the first month after a deal closes.
At a glance
- You don't always have to merge. Two tenants are fine if the businesses stay separate.
- Buy licenses in the surviving tenant during discovery. This is the most common delay.
- Start planning during due diligence, not after close.
Deciding what to merge
Do we have to merge the two Microsoft 365 tenants at all?
No, and sometimes you shouldn't. Two tenants work fine if the companies stay operationally separate, keep different brands, or answer to different regulators. Merge when people need one directory, one Teams, and shared files. Two tenants cost more in licensing and admin time, so the deciding factor is how integrated the businesses really become.
Can both companies keep using their own email domains?
Yes. But a domain can only live in one tenant at a time. The acquired company's domain has to be removed from its old tenant before it can be added to yours. That sequencing drives the whole cutover plan, because mail flow for that domain moves the moment the domain moves.
When should we start planning?
During due diligence. The tenant you're inheriting comes with its own security configuration, licensing commitments, and technical debt. The cost of fixing those belongs in the deal math, not in next year's IT budget. Our guide to IT due diligence before you buy a business lists what to ask for before signing.
Running the migration
What tools does Microsoft give us for this?
Microsoft documents a Migration Orchestrator for coordinated multi-workload moves, covering mailboxes, OneDrive, SharePoint, and Teams in batches. There are also individual cross-tenant migration tools for Exchange, OneDrive, and SharePoint, plus cross-tenant identity mapping. Third-party tools still fill gaps.
Do we need to buy licenses before we start?
Yes, and this one fails the migration outright if you skip it. Microsoft requires a Cross-Tenant User Data Migration license — "a per user license (one-time fee)" that "can be assigned either on the source or target user object" and also covers OneDrive. Microsoft's warning is unusually blunt: "Migrations fail if this step isn't completed. Microsoft doesn't offer exceptions for this licensing requirement" (Microsoft Learn).
You also need enough regular Microsoft 365 seats in the surviving tenant before migration. Order both during discovery. This is the most common reason a booked migration slips.
How long does a tenant-to-tenant migration take?
Microsoft's guidance is that "migration throughput depends on data volume, workload type, and batch size." The factors that matter are user count, mailbox sizes, OneDrive and SharePoint volume, mailboxes on hold, and bandwidth. For a small-business acquisition, discovery and prep take longer than the data movement. Budget weeks for prep, a weekend for cutover.
Can we do this gradually instead of all at once?
You can. Microsoft's guidance notes that for phased migrations you should "plan for a period where users exist in both tenants." That period is the cost: two sets of credentials, broken calendar lookups, and confused external senders. Phase it when the user count makes a single cutover risky. Don't phase it by default.
Compliance and the things that break
What happens to Teams chats and channel files?
Channel files ride along with the SharePoint sites behind them, so they migrate reasonably well. Private one-to-one chat history is the hard part and often doesn't survive intact. Tell people in advance that their chat history may not follow them, and give them a deadline to export anything they need.
What about litigation holds and retention?
Check before you schedule anything. Microsoft notes that mailboxes on hold might block migration, so discovery has to cover compliance configuration and not just user counts. If the acquired company is under legal hold or industry retention rules, bring in counsel early. Our data retention guidance covers setting the surviving policy.
What usually goes wrong?
Four things, in the order we see them:
- Licenses never purchased in the target tenant.
- A domain that couldn't be released from the old tenant on schedule.
- Shared mailboxes and distribution groups nobody documented.
- Line-of-business apps still authenticating against the old tenant's Entra ID.
That last one surfaces Monday morning, after the migration looked clean all weekend.
Inherited a Microsoft 365 tenant and need a plan for it? Request a complimentary migration assessment. We'll inventory both tenants, flag the holds and app dependencies, and give you a cutover plan with a real date. See our managed IT and cloud services for what the work covers.
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