Just because the pension accrual rates for PA change after 2015 doesn't mean that the PA pay spine disappears does it?
As for 'applying' it depends on rank. For Flt Lts offered assimilation beyond 16/38, or IPP or whatever it is now, that service will be as PA (just as it was as Spec Aircrew). Single list Sqn Ldrs are boarded for transfer to the PA spine.
Either way, it is definitely worth joining the Forces Pension Society. For those who will leave on AFPS05 at or before 55, there are some interesting comparisons between leaving at 55 and choosing to leave a smidge early - thus only receiving a reduced annual income until 65 but then benefitting from a second lump sum. Also some information about inverse commutation which is all about increasing your pension beyond 65 at the expense of the second lump sum - I think!