It is standard practice as per the AP3392 that if you either NGR or PVR, then you go down from the higher rate of flying pay to the middle rate. This is because flying pay is viewed as a retention incentive.
I suggest that you do what I will do when the situation happens to me. Give 28 days notice that you no longer wish to draw flying pay, and on day 29 refuse to fly until your service is terminated.