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.
What was the date of your commissioning. Reading between the lines, that's the contract you "signed" and entered into it. The bursary is given to you as an incentive to take up a commission and is not a binding contract on anyone's part, hence the ease in moving the goalposts for you chaps.
My sympathies to you all who got fragged by this particular knee jerk. I do consider it to be very unfair. And it doesn't encourage some of the more mercenary amongst the baby aircrew to push for the FJ stream, when the wait for the end of the OCU can be as much as 2 & 1/2 years later than their RW and ME cousins.
TTH