Well you need to repoint your MX records etc. as instructed on the Office 365 admin
set-up page and check that the records are set correctly using the wizard.
You can then migrate the rest of your mailboxes and shut down Exchange Server.
Then the federation services that are being used to connect to your system by Office 365 will only matter if you have some other systems using cross forest authentication.
Either way you can leave the ADFS.domain.suffix server running as it will not interfere with Office 365, or if you are sure you have no other applications authenticating cross-forest, you can shut it down.
