Office 365 hybrid, IMAP/SMTP on-premise, slow deployment possible?

I'm developing a migration plan to move from an on-premise SMTP/POP3/IMAP server to Office 365. I already have a few Microsoft Partner Program licenses for Office 365 and an account set up.

I have two employees fully in Office 365. The on-premise SMTP server is still our primary MX, forwarding the early adopter emails to the temporary @companyname.onmicrosoft.com aliases, and our Office 365 Exchange configuration has a connector back to our on-premise server so that the two employees on Office 365 can email everyone on-premise.

DirSync is working too, so those lucky early adopters can use their domain credentials to log into Office 365. Sweet. And it created users in Office 365 (though not everyone has licenses or mailboxes.)

Here's the question: the only remaining DNS thing to change is MX records, switching from our on-premise system to...