I was in that situation this year. I recommend telling your ROTC commander at the beginning of the year. I made it clear that while I hoped to attend USAFA the following year, I would remain 100% committed to the ROTC detachment throughout my year there. He told me that he isn't terribly pleased to be losing me next year, but he is supportive of my decision. Be honest with your CO about your future plans and inform him/her sooner rather than later. Odds are that he/she will be supportive of your decision.