Your terms of employment (for want of a better description) can't just be changed overnight by the MoD. Your terms are as stated in the relevant Pam(Air) xxx, xxx being the edition in force on the day you were due to swear your oath of allegiance. I joined on a Cadetship in 1987, the flying pay rules changed in 1989, and again in 1993. My flying instructional pay (that's going back some...) was halted, and the initial award of fg pay delayed. After much querying, 18 months worth of querying to be fair, the pennycounters agreed that my terms and conditons were as stated when I joined and I received fg pay back dated to when it should have started, which was a nice lump sum of around 8grand and several years seniority on the fg pay scale. vroom vroom.
It helped that I'd kept a copy of the Pam(Air) relevant to when I'd signed up, and had a friendly and knowledgable P1 staff instead of JPA staff.
If your bursary contained an offer of employment with a full commission in it, you have a case for claiming you're on the 'old' rules. If it was a bursary with no guarantee of employment at the end of it, I'm afraid it sounds as though you've dipped out. Serves you right for not going multi's
. Good luck.