There will be others who gained from the AFPS15 move from AFPS75 - particularly those joining the PA Spine around that time. This brings in career choices, ToS and RoS issues. It would be overly harsh to prevent someone from immediately pulling stumps on learning that their pension plan is not what they bargained for. So Option 1 would also be up for a direct challenge.
Option 2 would be rather extreme and legally dubious, especially for those who made career and financial plans on a pension that suffers a back-dated cut. I don't see how Option 2 could even be suggested as it throws-out any legislative precedent 'of sufficient time to prepare for a significant financial change' and would probably see a trip back to the courts. My guess is that Option 2 would just be a 'future plan' to move all remaining personnel (including those snapped-back to 75/05 terms again from the last fcuk-up) to AFPS15 terms at a future date.
Only Option 3 provides any legal certainty that the measure will survive scrutiny, as well as avoiding another own-goal when it comes to retention of more experienced personnel. Of course, the same clowns that pushed-through the last change and the stupid pension-tax burden are probably still around, ready to do their next self-dismantling pantomime car trick.