Never heard of changing a registration of a GPS unit on line (or otherwise). Since they are registered by serial number, and no names etc are required for updates, I don't see how the updater programs could tell who is at the other end, at least with my Garmin lifetime updates.
You should be looking new and buying one with lifetime map updates (its lifetime, not yours!!). Since features of the various units are changing all the time, mostly for the better, you would be much better off that way. Updates the old way by paying for each is cost prohibitive today. Units built into many autos are that way and map updates are ridiculously expensive....
I use mine all the time whenever I think I need it as they have much greater accuracy than the older ever models had. I never considered it be a "one time" need. I have had 3 over the last 30 years including 1 marine unit with NO maps. The current one is the best I've had so far. There is no comparison to using a phone unless you are really stuck IMHO.