Thanks for the fast reply BScout. I did in fact RTF*M. Hence my question.
With the change to Studio (and a doubling of tracks), there's seems to be a little more to consider than flattened folder tracks.
Unfortunately, the manual only discusses switching between closely related versions. Eg., page 490: Sharing Sessions Between Pro Tools 2022.4 and Pro Tools 2021.10 to 2021.12
Instrument tracks beyond 512 are lost, Audio tracks beyond the first 256 are lost, etc).
It doesn't say jack about sharing between larger version jumps (although logically it's not going to be all good news). Or maybe I'm missing something. No worries, though. I'll just hold off for now until I have a clear deck.
Thanks again! :)