I'm no expert but I imagine all your pre future pension scheme service will be based on your rank at the changeover to the new system. i.e. if you made sqn ldr before the changeover all your previous service would count as sqn ldr for pension purposes, or if you were Flt lt at changeover all service would count as Flt lt. The averaging would then begin, but you would have to stay in quite a while to get your 'average' in a higher rank. I'm no expert and this is assuming quite a bit, so take it with a pinch of salt.