Here is what is very likely:
- major hard attachment points (attachment to fenders/body) are probably the same.
- different center appliqué on the hatch is a minor issue that can be resolved easily in the aftermarket
- larger space for charge port for EU CCS combo plug is probably a priority
- since that requires a significant change to the taillight’s physical design, they will take the opportunity to update the overall taillight design to align with current design philosophy (diffused led, etc)
- since Tesla uses a bus to drive all electrical items now, if the old taillights don’t use that bus they will update to the bus in the new taillight.
- even if the current taillights do use the same bus, that won’t mean the new taillights will be plug and play with old cars since the lighting controller will minimally need different code for a different end point module.
- difference in charge port might be a major problem for plug and play into an old S.
Tldr: backwards compatibility odds are less than 50/50.