As far as I am aware, once issued with a service number, that stays with you for life. Ex ground airmen re-mustering to aircrew or officer or NCO aircrew to officer, took their numbers with them. The letter given before the number, on commissioning, moved to the back. It was mid 80's that I remember the letter being attached to the commissioned branches, even though it had existed but not used previously. Much to the consternation of at least one direct entry Ex V-force AEO sent to maritime, who insisted that that he never had a letter in his service number, and felt degraded by being referred to as an ex- NCO AEOp.
Last edited by cynicalint; 2nd Oct 2020 at 23:38.