When we were last visited by the AFPRB we pressed on exactly this issue. The reply was that earnt 'years in' would be kept, but that wouldn't necessarily be reflected in 'rewards earnt'.
i.e. As a AFPS 75 member, having been in 16 years and therefore earnt an immediate pension would translate to 16 'year credits' in the new scheme - but not necessarily an immediate pension.
As I am no expert I immediately joined the FPS.
Oh and promised to leave before it mattered.