Oggin,
I believe that once you've made your decision to stay on AFPS75 you won't be able to change to AFPS05 whilst still serving. If you left the services and cashed in on AFPS75, then rejoined, you would automatically be in AFPS05 - but you'd be starting from scratch again.
So you've got to think really hard about your chances of being offered PAS. At the moment, I hear that almost nobody is being offered it - they have probably worked out how much it's going to cost them! By my reckoning (roughly 500 PAS at an extra £10000 a year pension) inclusion of the PAS will cost about £5m a year - and that's just for the RAF!
I wonder if they took that into consideration when they did the sums to make sure that the overall cost of AFPS05 was neutral?