Dunno about the in-game Fantasia system, but if you want to change what the forums say you are, look at the upper right corner where it says "Welcome, DrMaverick" next to the (Log out) button. Click "Settings". Then "Edit Character". This will show you your available characters and allow you to set their top three jobs and select which character is your main character. [Save Changes] when done.

Your forum posts will show your main character (denoted by a gold star) and the primary job you selected (left-most slot).