Interservice transfers while in ROTC are possible, but you can't transfer one service's scholarship to another. They are all handled independently. You would need to withdraw from the AROTC scholarship and apply anew for the Air Force scholarship. If you truly prefer Air Force and feel fairly sure you would switch your sophomore year, I think the more honorable thing to do would be to turn down your AROTC scholarship, join the AFROTC unit, and apply for the 3 year Air Force scholarship during your freshman year. You would incur no penalty for rescinding your AROTC scholarship as long as you haven't started your sophomore year, but it would kind of be a shame to have the Army pay for your freshman year when you have little intention of completing your commission for them.