Merging two Microsoft 365 tenants after a merger or acquisition

Decide the end state first

Before any technical planning, the owners need to agree what the combined business looks like. Will everyone end up in one tenant, or will the two run side by side for a period? Will the acquired business keep its brand and email domain, or switch to the parent's? Which tenant survives is usually the larger or better-run one, but licensing, existing security setup and software integrations matter too.

Running side by side for a while is often sensible. Microsoft 365 supports cross-tenant collaboration features that let staff in both organisations find each other, share files and meet in Teams without merging anything. That gives breathing space to plan a full migration properly rather than rushing it in the first month after settlement, when staff are already adjusting to new managers, processes and branding.

Domains are the critical path

An email domain can only be verified in one Microsoft 365 tenant at a time. To move the acquired company's domain, it must be removed from the old tenant before it can be added to the new one. Between those two steps, mail for that domain has nowhere to go, so the domain move happens during a short, carefully planned cutover window.

Removing a domain from a tenant means first taking it off every user, group, shared mailbox and alias that uses it, which can take time in a busy tenant. Rehearse the steps on paper, prepare scripts where possible and allow for DNS changes to propagate. Mail sent during the gap is usually queued by sending servers and delivered later, but test and plan rather than rely on it. Tell key clients and suppliers about the cutover window so a delayed reply is not mistaken for silence.

Moving mailboxes, files and Teams

Mailboxes, OneDrive and SharePoint content can be copied between tenants using Microsoft's cross-tenant migration features, which need specific licences, or using established third-party migration tools. Either way, copy the bulk of the data ahead of the cutover while people keep working, then run a final pass during the cutover window to catch recent changes.

Teams is the trickiest workload. Channel files move with SharePoint, but chat history, channel conversations and app configurations do not always migrate fully, depending on the tool. Decide in advance what is important to keep. Often the answer is to migrate files, export or archive old conversations, and start the new combined teams fresh.

What changes for staff

Staff from the migrated organisation get new accounts in the destination tenant, which means new sign-in details, a new multi-factor registration, new Outlook and OneDrive setup on their devices and possibly a new email address. Phones need the old account removed and the new one added. Shared calendars and delegated mailboxes need reconnecting.

Communicate early and give clear instructions with screenshots. Plan support on the first working day after cutover, because many people will need help at once. Expect questions about missing calendar entries, auto-complete suggestions pointing to old addresses and Teams chats that did not come across. Keep a simple issue list and work through it.

The things outside Microsoft 365

Every app that staff sign into with their Microsoft account is affected: accounting software, CRM, document management, practice management systems and any service using single sign-on. Each needs reconnecting to the new tenant or having its users re-invited. Laptops joined to the old tenant need to be reset or re-enrolled into the new tenant's device management.

Licensing, billing and contracts with Microsoft resellers need sorting too, so you are not paying for two sets of licences for longer than necessary. Security settings such as Conditional Access policies, retention policies and sensitivity labels should be reviewed and aligned, because the two organisations probably made different choices that now need to become one.

Privacy and record keeping

Merging tenants means moving personal information about staff and customers. Under the Privacy Act 2020, the combined business needs to keep handling it in line with the purposes it was collected for and keep it secure during the move. Limit who can see migration reports, and make sure migration tool access is removed once the project ends.

Agree what happens to the old tenant. Usually it is kept in a reduced state for a period so nothing is lost, with its data backed up, then closed. Record when it was closed and what was retained. Tenant mergers are quoted in writing first, with a cutover plan both owners have signed off, because the domain window leaves little room for improvisation.