When doing a recent email migration project for Exchange 2013 to Office 365, I needed to remove the migrated mailboxes from Exchange. And I needed to keep the AD user accounts. The migration was not using Hybrid, and was done via cutover, using PST files.
Care is needed for this step, as the required step is to “disable” the mailboxes on Exchange 2013, and not “remove” them. If you “remove” the mailbox, then the AD user account is also deleted! The correct EMS cmdlet is disable-mailbox, and not remove-mailbox.
Watch out for this when doing an Exchange On-Premise migration to Office 365 via cutover. Deleting AD user accounts is a bad idea – use my tip to avoid it!