I think it was only last year when we all switched to the new JPA (Joint Personnel Administration) system which manages our pay on a Tri-Service basis.
Apparently, so I'm told, JPA cannot handle people of the same name, NI number & date of birth joining under a different service number so those that had a P0........x number get to keep it if they rejoin & I've heard of ex-URNU students joining on their old URNU service numbers & other RN or RM Reservists who join regular service keeping their old numbers. Whether the same applies for ex-OTC & UAS guys I'm not so sure.
The idea with the new numbers is that if you change services, you keep the same number.