Microsoft 365 Tenant-to-Tenant Migration: FAQ and Checklist

A Microsoft 365 tenant migration, also called a tenant-to-tenant migration, moves people's mailboxes, OneDrive files, Teams chats and shared SharePoint content from one Microsoft 365 organization into another, usually after a merger or acquisition. Microsoft's native tools need a one-time, per-user Cross-Tenant User Data Migration license, move content but not identities, and leave SharePoint sites and Teams channels to a separate tool. For a small business, expect weeks of discovery and preparation and a single weekend cutover.

When you buy a company, the IT question that takes longest to answer is what happens to their Microsoft 365 tenant. Below are the questions owners and controllers ask us in the first month after a deal closes, checked against Microsoft's documentation in October 2026.

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.
• Find mailboxes on hold early. Microsoft blocks their move.

Microsoft 365 tenant migration checklist

The order we follow for a small-business tenant merger, from the first conversation to the Monday after cutover:

1. Decide the end state. Merge, keep two tenants, or split one off, and name the people in each company who approve the users, domains and data in scope.
2. Inventory both tenants. Users, shared mailboxes, distribution groups, domains, OneDrive and SharePoint storage, Teams, guest users, devices and every application that signs in with Microsoft Entra ID.
3. Check holds and retention. List every mailbox on litigation or retention hold and agree with counsel how that data is preserved, because Microsoft's native tool will not move it.
4. Buy the licenses. Enough seats in the destination tenant, a Cross-Tenant User Data Migration license for each person whose mailbox or OneDrive moves, and any third-party tool licenses.
5. Prepare identities. Create the destination accounts, decide sign-in names, and map each source user to a target user before any content moves.
6. Plan domains and mail routing. Sequence the domain's removal from the old tenant and its addition to the new one, with temporary routing for the gap.
7. Pick a method per workload. Microsoft's orchestrator for mailboxes, OneDrive and Teams chats; a separate tool for SharePoint sites and Teams channels.
8. Run a pilot. Move a small group and test sign-in, mail delivery, files, sharing and Teams before booking the cutover.
9. Tell people what changes. The date, what they must do on Monday, and what will not follow them.
10. Cut over and support. Move the domain, run the final batches, check mail flow, and keep engineers on hand for the first working day; retire the source tenant only after the agreed review.

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's planning guide points to the Migration Orchestrator for coordinated, multi-workload moves in batches. Its own documentation limits it to four per-user workloads: Exchange mailboxes, OneDrive, Teams chats and Teams meetings. Shared data, meaning Teams channels and SharePoint sites, stays in the source tenant. There are also individual cross-tenant migration tools for Exchange, OneDrive, and SharePoint, plus cross-tenant identity mapping. Third-party tools still fill gaps, especially for SharePoint sites and Teams channels.

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. It is sold as an add-on to Microsoft 365 Business Basic, Standard and Premium, the enterprise and frontline plans, and Office 365 E1, E3, E5 and F3. 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).

SharePoint sites are licensed differently. Microsoft sells its Cross-Tenant Shared Data Migration licenses per 100 GB moved and, as of October 2026, only to Enterprise Agreement customers, which leaves most small businesses using a third-party tool for sites. You also need enough regular Microsoft 365 seats in the surviving tenant before migration. Order all of it 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. Native OneDrive and SharePoint moves are one-time moves with no incremental or delta passes, so they belong in the cutover window rather than in an early pre-copy. 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. Microsoft's orchestrator documentation describes a single-event migration as best suited to small and medium businesses. 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?

Chats and meetings can now move. Microsoft's orchestrator migrates Teams chats and meetings along with the mailbox, though participant lists change and some threads can appear in both tenants. Channels are different. Teams channels and the SharePoint sites behind them are shared data that the orchestrator leaves in the source tenant, and Microsoft's own cross-tenant SharePoint tool moves only the site content, not the Teams channels or their structure. It cannot merge into an existing site, and flows, apps and workflows have to be rebuilt in the new tenant. Tell people in advance what will and won't follow them, and give them a deadline to export anything they need.

What about litigation holds and retention?

Check before you schedule anything. Microsoft's cross-tenant mailbox documentation says mailboxes on any type of hold aren't migrated and the move for those mailboxes is blocked, and that the source mailbox is deleted after a successful move. Discovery therefore 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 and settle preservation and recovery, including whether to keep a Microsoft 365 backup of the source data, before anything moves. Our data retention guidance covers setting the surviving policy.

What usually goes wrong?

Four things, in the order we see them:

1. Licenses never purchased in the target tenant.
2. A domain that couldn't be released from the old tenant on schedule.
3. Shared mailboxes and distribution groups nobody documented.
4. 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.

Frequently asked questions

Is an Office 365 tenant migration any different?

No. Office 365 and Microsoft 365 subscriptions live in the same kind of tenant, and the same tools, identity mapping and license apply; Microsoft sells the Cross-Tenant User Data Migration add-on for Office 365 E1, E3, E5 and F3 as well as for Microsoft 365 plans. What changes the project is the workloads and data in scope, not the name on the subscription.

What is included in a NetSys tenant migration?

Four pieces of work, scoped before anything moves. Discovery covers owners and approvers, users, domains, mailboxes, OneDrive, SharePoint and Teams, business applications, guest access, holds and acceptance criteria. Identity and domain planning maps every person and address to a destination and sets up temporary migration access that we remove afterward. Method selection compares Microsoft's native tools with third-party options for each workload and proves the choice in a pilot. Cutover covers migration waves, user communication, mail, sign-in and file checks, recorded exceptions and transition support, with the source tenant retired only after the agreed review. Details are on our Microsoft 365 tenant migration service page.

How does a tenant migration project start?

With a short call. Tell us whether it is a merger, a consolidation or a move, plus the approximate user count, workloads and deadline, and we scope the discovery needed to plan the migration, licensing and support. Administrative access to both tenants is then arranged through an approved, secure process; never send passwords through a contact form. If the deal has not closed yet, our IT due diligence review can size the tenant work before signing.

What affects the cost of a Microsoft 365 tenant migration?

User count is one input. Data volume, the workloads in scope, existing accounts in the destination tenant, domains, application dependencies, permissions, retention requirements, the migration method and the support people need all change the work. A useful quote separates discovery, migration labor, Microsoft or third-party migration licenses, destination subscriptions, application remediation and transition support, so you can see which line moved. A mailbox-only move and a merger that includes SharePoint and Teams are different projects with different prices.

Who handles support during and after cutover?

NetSys engineers. Escalation contacts on both sides are agreed in the plan, and over cutover weekend and the Monday after, our help desk handles sign-in prompts, apps that need reconnecting and line-of-business software still pointing at the old tenant. The help desk is staffed seven days a week from 4 a.m. to 11 p.m. Eastern, and emergency service runs 24/7. After the transition period the surviving tenant can move under ongoing Microsoft 365 management.

Planning a tenant migration with NetSys

Book a call if you have inherited a Microsoft 365 tenant and need a plan for it. We'll inventory both tenants, flag the holds and app dependencies, and give you a cutover plan with a real date. Our tenant migration service covers the project, and our Microsoft 365 migration page covers broader platform moves.

Sources and further reading

• Plan a Microsoft 365 tenant-to-tenant migration, Microsoft Learn (updated June 2026).
• Migration orchestrator overview, Microsoft Learn: supported workloads, licensing and what stays behind (updated August 2026).
• Cross-tenant mailbox migration, Microsoft Learn: the license requirement and holds (updated September 2026).
• Cross-tenant SharePoint site migration overview, Microsoft Learn: Enterprise Agreement availability and limits (updated May 2026).