My experience is, which is over 40 years gone nigh, you are pretty much stuck with what you signed up for. The Navy is a bunch of bureaucrats that are programmed to nothings according to procedure. If it is not SOP, then it requires special processing. And I'm sorry to say this, but in the military, you are not special, not even if your name is Forest Gump. My advice is "sign up only for what you want, 'cuz there are no redo(s)."