Yes, AEC is open as are probably all occupations at this time of year. The problem may be that you may not be able to switch. If you are in BOTP now, you were given and accepted an offer for MARS(from your post). When you are in BOTP they usually won't entertain requests to change occupations. They are very busy in St Jean and are they to ensure people get through basic. You may have to finish basic, go where ever they send you (some base) and then request an occupational transfer.
If you are seriously considering it you should talk to your Directing Staff now, don't wait till the end.