There are exist two ways to migrate mailboxes:
Way 1:
- Migrate all mailboxes to Exchange online, then decommission the old Exchange on-premises.
- Create hybrid and AAD sync with new domain/forest, migrate mailboxes back to Exchange on-premises, if you want.
Way 2:
- Migrate all mailboxes to Exchange on-premises, then remove hybrid and AAD connect.
- Use ADMT to do a cross-forest migration to migrate mailbox from old domain to new domain. You could have a look about this article(Suitable for Exchange 2019) Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
- Create AAD sync and hybrid in new domain.
If there doesn't exist hybrid in your organization, I think the second method is more suitable.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.