The old owner deregisters the mixer (hardware) and the new owner registers the mixer new. PreSonus has kept this process very easy and user friendy. Just do it... ;-)
Things are different with software, because software is regarded as being connected to a person/owner. But a transfer is possible. Transferring a Studio One licence costs about 25 Euro (which is not much compared to buying Studio One) and I suppose something similiar is going to happen when Fat Channel plug-ins will be transferred.
But this is a new theme. I have never seen it appearing anywhere in Forums, Answers or Facebook. The best way to get an answer would be to open a support ticket, which only can be done when you are a registered owner of hardware. Please report back. The question is getting more and more relevant.