It appears that Garmin is under no obligation to authorize any device change for the bundled Mobile PC and City Navigator, including any and all additional maps and subscriptions purchased. It is for all practical purposes, locked to a device, period. If the device is lost, stolen, or destroyed, everything "soft" becomes worthless. That makes a working device worth what software and subscriptions the owner has added to the device, plus the cost of the device. A $50 piece of plastic could easily have over a $1000 invested in it.
In the case of a broken, damaged, destroyed receiver that could be identified by S/N, it appears Garmin will overhaul for $49 for the 20x. I would expect Garmin repair department to return me the "same" association with my account. As long as they service my model and I can return at least the serial number, it looks like I'm good for the lifetime subscriptions.
In the case of the lost or stolen receiver, I would expect Garmin to say "We're sorry, but we cannot associate a new receiver with that account" and that entire subscription is dead and the owner is out of luck. What have others experienced?
In the case of just wanting to change from one receiver to another, we're probably out of luck, too. I don't see why Garmin would even bother, when they don't have to. I would really like to change to a 10x, but I've already bought extra maps all on the lifetime deal. Has anyone ever changed from one device to another, using bundled Mobile PC? What happens when you try to introduce a new device?