Whatever they do it will cause moaning... Genuinely can't win.The shareholders association have asked this question in one of the recent quarterly meetings with the club.
I think it was initially stated that the club would try to do an almost straight transfer of like for like for whole corresponding sections. Forming a best fit of those 30k+ season tickets to the corresponding sections in the new stands. The obvious problems there are that the stands don't really match up perfectly and new corporate sections are much larger and fill some areas that are currently general admission. On top of that both Gwladys Street and Park end season ticket holders might favour the large South stand because the North stand might be less desirable or feel a bit non-descript by comparison. Similarly, fans currently in upper tiers at GP might prefer lower tiers at BMD to be closer to the action. I know that similar moves elsewhere have led to some quite contentious decisions with many fans left disgruntled.
I think the outcome of the discussion was that they'd go for a best fit with some opportunities for choice throughout the process. Not sure how that pans out in practical terms.
They're going to struggle with family enclosure/main stand/balcony because as you say, the new West stand has masses of premium seating.
Ideally, they'll heavily incentivise first year premium sign ups so they're all squared away early and they'll know exactly what they're working with for the remainder of the ST holders.
If I were doing it, the priority would be:
Shareholders
Season ticket holders on years held
New season ticket holders
If the premiums are confirmed 4 months ahead of the move, they can give shareholders a week, 10+ years ST a week etc all the way down to about 6 weeks out from matchday 1.
It's plenty of time. (if organised properly and not from a printed excel sheet)