I've probably said this before, but I think that any upgrade path that requires any non-trivial amount of engineering work is just not going to happen. We can come up with all sorts of clever technical ways of making MCU2 drive IC1 the same way as IC2, etc. etc. etc. but any approach that requires product development is going to be seen as a distraction from other things that Tesla has to do to ensure its survival and long-term viability. Moreover if these upgrades do happen, they'll be creating new, little-tested car configurations, and I'd rate the potential for bugs higher compared to standard factory builds. I know that Elon has said it's possible, but just saying it doesn't make it so.
Just my opinion. I have no insider information, and I hope I'm wrong.
Bruce.