To qualify for a full pension one needs to serve either 2 yrs substantive or 3 yrs acting. However as long as you serve at least 1 yr in your new rank then you'll receive a factored difference between your previous rank and the rank you now hold.
If you are acting then for each yr served in that rank you'll receive 1/3 of the difference (ie full pension after 3 yrs) and if you are substantive then 1/2 of the difference (ie full pension after just 2 yrs).
The HR staffs hold the AP leaflet that refers to this (sorry can't remember the number)
Cheers