It seems to me that you would be no different than any other "college applicant" (if you got out of your current commitment). If you tried, I would like @kinnem bet significant body parts on not being able to switch services mid stream. Even if by some miracle you did manage it, I would assume you would start at "day one". A better idea may be to graduate, then try to switch services. I know that has been done in the past, ie: aviation to aviation, sea service to sea service, medical to medical, engineering to engineering, and legal to legal.
the Academies have an Exchange Program where Cadets go to another academy for a while. And you can ask to cross-commission after graduation. Not sure what you’re asking.