@robparsons there's a garage in the Netherlands that does it, "De EV Garage", and they basically just open up the original battery and fill it with brand new higher density cells, and make the pack slightly bigger by installing standoffs and sealing the pack all around. It does make the car heavier (~150kg), so you need to reinforce the suspension.

All in all it's an expensive upgrade (10k for the pack alone), but if you compare that to other 70kwh new cars it's still a very good deal I'd say!