I infer from the question and the forum that the OP is an ROTC cadet who just got his branch and component assignment, and not an enlistee. My answer revolves around this presumption, and will thus have little value if I've guessed wrong.

You can change your USAR assignment to a National Guard assignment, but you must move quickly and decisively. Contact the OSM of your future state of residence, and ask for an acceptance letter with a specific unit, with a specific paragraph and line number. Since the combat arms branches have the most lieutenant slots, you're aiming at a big target (which is good). Before you make the call, consider realistically your branch preferences, and how far you're willing to drive. I would recommend no more than three hours each way, but that's your call. At the same time, keep your ROTC chain of command informed as to your progress. The OSM will have some requirements for you to complete, none of which are particularly difficult. It gets very difficult, however, if you don't cooperate.

Remember, this is your life, and no one will take it any more seriously than you do.