I think Season 1 Final is the lock.
Once we start Season 2 - there should not be any major character body changes that are not explainable by in AVN events or trends (like the MC getting more buff as he puts in the work at the gym and on the court).
I could see the occasional hair style change (especially among the ladies) and I understand that lighting can/will wash out character's faces the same way it does in RL.
Some caveats:
Software updates and changes will impact the character models we are used to - Blender versus Daz for example.
We have not had any sexy time physics yet - movement of hair, breasts and butt during said time - and we won't know how/if the physics requires any modifications of the models to allow the sexy time to be well-rendered/animated.
Moving to a higher level of fidelity - like 4K - could reveal issues with models that were not visible/evident at the current resolution.
I trust Ocean to make the changes necessary to ensure that the move to increased image fidelity is as good as it can be while avoiding /correcting those issues.
I am definitely looking forward to the new scenes in CH 2, like the early BB scrimmage - and hoping that Bella can still hip check like a pro!
Cheers!!