I am pretty sure it is. Stealing something that someone legally bought from you back again because you made a mistake when calculating the sales price is blatantly illegal, regardless of whether the "stealing process" includes breaking up the car door and ripping out seats or unchecking some checkbox in a remote management software.

The latter is much easier, but not an iota less illegal from the viewpoint of the law.

Which is why the last car I bought, I intentionally chose something that had practically zero software.

A decent lawyer will make the case that it either is or Tesla should be in the hook to renegotiate every upgrade, every time the vehicle resells.