SW recruiters have two boards a year for Prior Service applicants. One is in the beginning of the FY and the other during the winter months (FEB, MAR) in hopes to ship everyone during that FY. It is difficult to get in if you become E-6 before you try to cross over. I would try and talk to a usaf special operations recruiter and see what kind of paperwork is involved but it is definitely possible. They might even be able to transfer your service commitment from Marines to Air Force earlier.