It follows the car if you sell directly to another person or through a third party dealership. If Tesla gets the car back, they'll wipe the upgrade and charge for it again.
It won't happen until cars can talk to each other, think of how ants walk in a line and follow each other around an obstacle.
Car A (driven in manual mode) tells Car B (FSD) that tells Car C (FSD) that tells... Car Z (FSD):
"I stopped at a stop sign in stop & go traffic at Lat/Long xyz while I spotted a human in the middle of the road. This is marked on my maps data (updated overnight) as not being a normal stop. No new data was detected except the human standing and a human laying on the ground."
Car B, C, D... all confirm the same until Z passes the area and sees no more humans (they've been moved). Car Z tells Car AA all good here, proceed as normally.
I'm just rambling now, but it's truly how ants operate. Draw a line of chalk in the middle of their "line" or walking patch and watched them avoid it and find a way around it. The difference is all of these cars will feed a true neural net of public data of live road conditions, accidents, etc. A possible privacy nightmare, but these are different times.