When I migrate customers to Exchange Online I always recommend they maintain an Exchange hybrid server. There are several reasons for this recommendation; it’s much easier to manage SMTP addresses with an Exchange server on-premises, and it’s much easier to get your mailboxes back out of Office 365 if you maintain a hybrid server. To me, one of the great advantages of Office 365 is that I have never felt Microsoft was trying to “trap” customers in the service. They have always made it fairly easy to move mailboxes in and out of Office 365.