Exchange to 365 Migration Questions

[Todd Purifoy]

If there is a better place to direct this question, please let me know.

We recently acquired a small company and they have Exchange on-prem.  They have their own domain (oldcompany.com), and we'd like to migrate their mailboxes over to our tenant.  In our tenant, they have a mailbox with OUR domain attached (newcompany.com).

Most of the methods appear to create a new user and mailbox.  

We just want to migrate their existing mailboxes over and merge them with the mailboxes on our side such that they would end up with BOTH SMTP addresses.  Then we could flip their primary to oldcompany.com and they'd be good to go.

There are only 54 mailboxes on-prem.

We already have a domain user for them, and a mailbox.

Using the hybrid or cutover appears to create an entirely new user?

Almost seems as if manually dumping to a PST and importing on the 365 side would be just as easy considering the small amount of mailboxes?

Any suggestions?

[#2]

Well technically, it will be a new user as those are different systems. However, the user object will be linked to the on-premises one, and the mailbox can then be "attached" to it. The benefit of using Hybrid is that there will be no need for the end user to reconfigure their profile, but for just a handful of mailboxes you might indeed go the manual export/import route.
You cannot use either Hybrid or Cutover if an Office 365 mailbox already exists for the user.