We are planning on doing a cutover migration from exchange 2010 to office365.
for example we have about 100 E3 license but we have about 150 mailboxes onpremise, the extra 50 are not needed for production but will be needed for the client to refer back sometimes( these are mailboxes of users who have left the organization but their mailboxes are still required). i understand that once the migration batch is started all mailboxes and accounts get synced, so at the end I will have 150 mailboxes on the o365, I activate 100 of them, what happens to the remaining 50 if we dont assign a license to the accounts, do they just stay there forever until we decide to get additional license to activate them?