If history serves, it will not be Tesla's policy to upgrade a car.
But as a thought expeciment: As I understand it, the rear unit is a different setup so the would require replacing. It's possible the cabling is different to handle higher currents, then there's the much discussed fuses which may or may not be different from the 85D. The software's different too, but for all we know it could be the switch of a few bits.
That said, I'm sure it's possible to do with enough money, knowledge, parts and equipment, but it'd be more cost effective to trade.