It's very difficult to say. A lot depend on to which specialty you're trying to switch. If you wanted to switch to psychiatry or family medicine, then your chances are good. With any sort of intermediate or highly competitive specialty, then it's more bleak. Also, because the Army match is relatively small, it is more susceptible to statistical anomalies than the civilian match.
Overall, the military match tends to "punish" indecisiveness. First of all, in the Army, many specialties earmark residency slots for medical students. If you were to re-apply as an intern, you are no longer in the same applicant pool as you were the year before. Instead, you are competing against GMOs, other interns, and those looking to do second residencies. Secondly, because the match is so early in the year, you will not have much time to 1) decide to switch and 2) more importantly, act upon that decision (e.g. rearrange your intern schedule to get the appropriate LORs).
