If you are still in Phase 2 then it's not retrading it's Re-allocation and there's no timebar on this from phase one. You will find this won't take very long as you're not much use as an unqualified soldier.
The only time bar is a return of service (to the Army) time bar and not a time bar in the corps, so in short, you can Apply for a voluntary transfer (AFB241) whenever you want. If you've just finished tech training (and probably got your lance jack as a result) you can retrade immediately, but expect to lose your rank. Its even easier to get as a tech than as a clerk!
Having read many of your posts, I just have to say that you come across as a patronising little sod who gives the impression that he knows it all, but actually knows very little. Were you never in a position of only being in the field force for 9 months, Chuzu is asking a straight forward question, to which I do not know the answer and neither do you by the looks of it, so why post in the first place?
To get back to the original question, if you are unhappy with your trade then there is no rule stopping you from retrading even after 9 months in field force. Platoon commander is your best bet like I said. He'll probably refer you for an OC's interview and it'll go from there.
I would have thought that the first port of call would be the RCMO, simply for a bit of advice. Pl Comds know very little themselves of Army life and for them to simply refer you for an OC's interview is not particularly constructive and might put you off.
I understand that the REME have a shortage of reccy mechs at the moment? If in an LAD, the unit you are attached to should have an RCMO that you can approach for advice - if he/she cannot help, then at least they can point you in the right direction.