@nvah You are going to have to do this internally - i,e it has nothing to do with a AFCO. First off you need to be aware if you are
over the 6 month point you " may" hit some issues. As you have not passed phase 2 you are not on GTS (gained to trained strength - and will not yet have a return of service for your current branch) if you are in a shortage branch the Navy is under no obligation to release you even to the RM until you hit 4 year point on release. As lets be honest here you have wasted a considerable amount of peoples time and money if you were to just swap over!!.
i,m sure you re-call the contract you signed and went through at least 5 times with your Careers Office and at HMS Raleigh - before you signed.!!
You will need to speak to you instructors / DO and then it will be a chat with the Officer I,C your training branch / Dept and you will have to put you best case across. It is do,abale and some ranks have done it , but it is not a given and as above if you are in a shortage branch they may be reluctant to grant you a change. and if you think RN basic (it will help) has prepared you for RM basic fitness wise - it has not.!
Best of luck .!!