The OP's story is a bit unclear.
Did you originally ask for a nom to USNA and now have changed your mind or did you ask for both and only got USNA or did you "rank" your choices and the MOC gave you USNA?
The reason I ask is that, in the first scenario, the MOC didn't know you wanted to attend USAFA. If he has not sumbitted his slate and there is someone on the USAFA slate who really wanted USNA, there is a VERY SMALL possibility that the MOC would switch you and that other person so that both are "happier." However, it is a VERY small likelihood.
If you asked for both and/or ranked them, the MOC has probably already considered you for USAFA and decided to give you USNA. Thus, asking the MOC to give you USAFA is not likely going to have a good outcome b/c the decision was already made. If you ranked USNA ahead of USAFA and now have changed your mind . . . you're also probably out of luck.
The one caveat is if you are in a non-competitive district/state where the MOC may not have filled all of his/her nom slots. So, if the MOC only has 8 nominees to USAFA and you're now telling the MOC for the first time that you're also interested in USAFA, the MOC would likely give you that nom as well.
I don't think it makes much difference whether you're a principal nom. Presumably, if the MOC uses the principal nom system, he/she has already made a principal nom to USAFA. The MOC isn't going to take it away from that person nor is the MOC likely to drop someone from the USAFA slate once the MOC has told those people they have noms.
As you can see, as a general rule, it's extremely difficult to get a nom changed. This is why, in competitive areas, it is critical to choose wisely when ranking/selecting SAs to which to seek noms. Not saying the OP didn't do that, just noting that decisions have consequences so best to make those decisions early on, difficult as that may be.