Depending on whether you have passed all the required tests for Pilot and still get WSO, it is possible to re-muster. But...
Wait a few years to do it; the RAF would have trained you up in WSO and ploughed millions in to it in doing so. It is possible to do it but don't bank on it. At the end of the day it is up to them. I have heard of someone going in as WSO and at 25 go through Pilot training. He now flies Civvy after just 8 years flying for the RAF. Again it is possible but by all accounts fairly uncommon. If it's really Pilot you want though, why not apply for the FAA aswell and see where you get with that. It's all fine being WSO but if your hearts not with it and all you want to do is fly then it will show on the surface.
Good Luck all the same mate
Dave