How Can Companies Plan a Microsoft 365 Tenant Migration When Both Tenants Must Stay Active?
During a merger, acquisition, or business divestiture, it is not always possible to move every employee to a new Microsoft 365 tenant on the same day. Different departments may have separate deadlines, applications may depend on the original tenant, and employees may need to collaborate with colleagues who have already moved. This creates a coexistence period in which both tenants remain active.
The most important step is to define how users will work during this transition. Administrators should identify which users are moving in each phase, when their mailboxes will be transferred, and how email will be routed while the migration is underway. They should also plan how users in different tenants will communicate, access shared files, and schedule meetings.
Calendar availability is one area that can be overlooked. Employees may need to see whether colleagues in the other tenant are free or busy before scheduling meetings. Cross-tenant collaboration settings and calendar-sharing arrangements should therefore be reviewed before the first migration batch. Teams communication, external access, and access to shared documents also need to be tested rather than assumed to work automatically.
Another important consideration is the timing of domain changes. If the organization intends to move a custom email domain between tenants, administrators need a carefully sequenced plan for domain removal, verification, DNS updates, and mailbox routing. Changing the domain too early can disrupt mail flow or interfere with accounts that have not yet been migrated. The sequence should be documented and tested with the relevant IT stakeholders.
The SysInfo Microsoft 365 Tenant to Tenant Migration Tool supports migration of selected workloads, including mailboxes, OneDrive, SharePoint, and Microsoft Teams. Administrators can map source users to destination accounts, process migration batches, and review reports. The exact workflow and supported components vary by workload, so the coexistence plan should account for any features that require separate configuration.
Steven Miller
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