Is this a material change, as defined by a reasonable person? Probably. You have a well established and adopted approach to widely available, industry standard functionality replacement with a proposed, unproven approach.
How long does it take for a reasonable person to realize that the change is material? I would argue rather quickly, within a few days/weeks of delivery, but certainly more than a few hours/miles. Probably have a case that one needs to use the car a bit to realize.
Is there evidence of efforts to cure and what is the reasonable expectation of time to cure? Two aspects of this. On the surface, one would argue that Tesla knew at least 6 months ago that they would stop delivering that functionality and in the last 6 months there is little outside evidence they put efforts into cure. The firmwares they issued do not even have evidence for attempt in that. This will probably require evidence of internal documents (plans, code) and discussions. The seconds aspect is about if such cure is possible, within reasonable timeframe, with their approach. That would require experts and will quickly become very expensive to argue.
The answers to those questions are certainly not clear cut and they are, at least, worth pursuing in the legal space.
On the other hand, interesting things are happening at Twitter that may have positive effect on this whole thing, albeit not immediately.