I would be very reluctant to go through IOT as an FC on the off chance that you may be transferred to Pilot afterwards. This is very rare in my experience and is highly unlikely to happen when they are not exactly desparate for Pilots. Guys who get a lot further down the line than UAS are not automatically given another role in the Pilot branch if they are unsuccessful in a particular stream, let alone other branches.
If you are keen on being an FC and are prepared to spend your 6 years or whatever it is they offer you in that role, then go for it. You have to be careful though, in my opinion there would be nothing worse than going through the process of IOT hoping for a branch change and assuming it will happen. You would be much better off pursuing a civvy flying career or reapply at a later date providing you meet the age criteria. It really comes down to one simple choice, do you want to be a Pilot or do you want a career in the RAF?
I know that when I was at IOT there was the opportunity to transfer branches toward the end of the course, not that this ever happened it seemed, but you could be considered. I would put money on the fact that this process has been removed now?? any one from DIOT care to shed any light??
Good luck