In a wave-based migration, you pay for licenses in two tenants from the first cutover to the last. Users who have cut over need active Exchange Online licenses in the destination tenant. Users who haven’t migrated still need active licenses in the source tenant, so both license pools must stay available for the whole wave period.

That overlap hits the budget, especially in acquisitions where the goal is to cut license spend after consolidation. Source tenant licenses can’t be cancelled until all users have migrated and the source tenant is decommissioned. If you remove source licenses early, BitTitan loses the source-side mailbox access its migration jobs depend on.

Estimate the overlap from the full wave timeline, first cutover to last. In a three-wave program spanning 12 weeks, for example, the dual-licensing period is roughly 12 weeks. Put that cost in the program budget up front instead of discovering it after approval.

Have you calculated your dual-licensing overlap and put it in the migration budget?

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