Basic point: If Tesla comes into actual possession of a Tesla, as in, “We’ve got title!”, then they can do whatever they want with the car. Up to and definitely including removing any paid-for add-ons, such as FSD, EAP, acceleration boost, and possibly free-for-life Supercharging. (I think I’ve heard some arguments about whether that last is tied to the original owner or the car.)
Some time back some punters bought used Teslas that, indeed, at the time they were bought, had FSD installed. And the punters paid for the cars with that expectation. For some reason, after the punters had paid for the cars and received title, Tesla reached out and removed FSD with no warning. The news of this reached several outlets, class action lawsuits were being readied, but finally somebody sane within Tesla realized what was going on and restored FSD to all the affected.
Nobody’s quite sure, but it may have been that some Tesla service centers were either going over the cars pre-sale for the used car dealers; or these were used cars owned by Tesla that they forgot to clear FSD off of; or some other situation.
But if one finds a used Tesla for sale, in or out of a Tesla facility, and that phrase, “FSD Included Package” is in there, then the package is in there and locked to the VIN. Just like the paint and anything stowed in the trunk.