New Acquisition - migrating to O365

the amount of users is indeed a factor to take in to consideration.

however, the identity part for me would make the difference.

- are you using AADConnect to synchronise your ad-objects?

- will you migrate the ad users in to your primary domain? (admt, ...)

what i would do is the following : 

- make sure that the ad-objects are present in your primary domain (or by migrating them, or by just creating them)

- make sure to fill in the necessary details (not the currently used e-mailaddress)

- synchronise AADconnect

- configure your HCW on the Exchange 2010 server

- add the @tenantname.mail.onmicrosoft.Com as an alias to your mailbox

- and migrate all mailboxes to Office 365

- once all mailboxes are migrated you can add the domainname, and if needed change the primary address

these steps are quickly written down, please forgive me if I missed a step.

good luck, 

Tom