Just thought I'd resurrect this to post the following:
Long story short - an OEM retrofit of the powered boot from the 2021 on cars isn't going to be practical. At the very least the hinges have changed to accommodate the larger strut, as well as a mounting point for the hinge on the actual bootlid. Raymond @ TeslaOffer seems to think there are changes in the wiring looms too.
Obviously TO have a vested interest in people buying their products vs getting an OEM retrofit, but the hinge difference is as clear as day.
A ranger who came to do remedial work on my car twice was adament that an OEM retrofit was in the wings, based on the number of enquiries the SC had been getting, but I can't see how it would be remotely possible for Tesla to do it without significant work (particularly if there are loom changes involved), and based on the glacial timings involved in even simple things (e.g. allowing MY21 SR+ owners to unlock their heated steering wheel) I would not expect them to offer this on any practical timeline, even if you assumed it was possible.