Well, I would say don't mix SSIDs on differente wifi gens (which is not quote the same as the same gen AP by the way) - you COULD get a different AP in an out building on a completely different SSID and it should be no problem at all (similarly I have an AP set to a different SSID completely for in-laws to use when they're visiting with their trailer - where they put it is a dead spot for wifi (and no don't care to have it there any other time), and I wasn't investing in a matching AP, so used one of my old ones on a 2.4 only SSID)
Honestly, 'they' don't need anything. You log out of things like SiriusXM/Deezer etc etc from any interface. Unregister controller is done from customer.control4.com, though frankly, the moment the controller gets registered to a new account, your connection to it is dead, so unregister of the system is optional (though not the services for music)
Cyknight's post in Unifi APs Dropping out of Controller but Staying "Blue" was marked as the answer