As far as I know, this is currently not possible.  In particular, there doesn't seem to have a way to download and install a new device agent to cleanly replace the counterpart in the orignal MDM system.

If they were previously enrolled into another SOTI environment under AEDO then you should be able to enroll them into your environment as the agent should still be registered as the DO. If they are from another MDM and there is another agent set as DO then no, they will need to be factory reset. I would honestly probably factory reset them anyway so that I know what state that they're in rather than inheriting some bad configuration that I don't have visibility to. 

Thanks Raymond.  That was the answer I was expecting. But it never hurts to ask.