Traditional car companies generally offer incremental improvements in the next-in-line version of an existing car. For example, while Ford has been incrementally improving its Mustang since 1963, the car still espouses a design philosophy similar to the original.
According to Musk’s tweets, there are unlikely to be any retrofits — i.e. your old Tesla car’s hardware would not be upgraded to make it come to par with the new version of the car, even if you are willing to pay for the same.
Here's the real sticking point: These auto companies, and auto component manufacturers, use long lived platforms as a way to spread design costs out over time, and also, to do the long term validation work that it takes to not produce rolling piles of unsafe junk.
We will see what happens.