Speak to your ASOM, they will give you advice and will know who your Career Manager (CM) is.
You have a training return of service from Phase II. If you haven’t served for long you may be required to complete this.
Changing trade is purely at the discretion of the RAF, specifically a combination of the manning levels of your current trade and the one you wish to transfer to. Whether you are released from your trade is up to your CM and Branch & Trade Advisor. Your chain of command also have influence on releasing you from your current JPAN prior to your FAD.
Requirements for specific trades in need can be found on the manning website on sharepoint, ask someone more senior to you if you don’t know where to find this. Last time I looked firefighter re-trades were the most in need.
After you’ve done all this and taken the advice of your ASOM. You or they on your behalf will contact the CM.