Hi TPIAdmin,
You do not need to delete on premise mailboxes which are migrated to cloud, and still can use them in your workspace. Once the batch is deleted, it will not sync between the on-premises Exchange mailbox and Exchange Online mailbox. There will be a 30-day grace period for you to assign licenses when the mailboxes have been migrated to the cloud. After the grace period, the mailboxes on the cloud will be deleted.
Thanks,
Johnny Zhang