The PAS pension supplements under AFPS 75 would be the accrued rights in this case, therefore, the individual on that scheme would probably have to complete the 5 yrs RoS to access the supplements earned before the introduction of the FAFPS.
With FAFPS being a career average scheme, those being offered PAS need to weigh up the potential benefits of having all their pension rights up to the new scheme's introduction protected and then starting on a career average scheme at a relatively high starting salary with future incremental progression available. Obviously its relative and future value will depend on what accrual and revaluation rate is agreed with HMT, but it may not be such a bad place to be when the new scheme is introduced.