In my experience, if you REALLY don't want to be anything other than pilot, don't join up as anything else expecting to get crossed-over. I commissioned as a nav in 2000 and it took me 4 and a half yrs to cross-over (and that was a fluke quite frankly). Having said that, I never regretted a moment...
On the x-over thing, I tried during and after IOT (having had a very good IOTC) and OASC sent me a very snotty reply which said (in precis) that THEY were the selection experts and that their original logic of selection still remained (I scored 165 for pilot). Don't believe anyone who suggests joining as one thing and then branch-changing at IOT - you could and probably will get your finers burnt.
Whatever you do, good luck and stay positive.
PS