Specifically, will Tesla upgrade any customer's car (not tesla employee/testing car) initially produced with a hardware 3 FSD computer to be able to run either unsupervised FSD, or the same-or-better model as what hardware 4 cars are running at that time?

I would guess that Tesla will not do this until unsupervised is already solved, but if Tesla upgrades the hardware to have full parity with hw4 but hasn't solved unsupervised on hw4 yet this will resolve yes.