I was in the exact same situation a couple of weeks ago. I had put USAFA as first choice, then had a visit at USMA and realized I needed to switch. You might go about this differently with a different MOC and different circumstances but what I did was I reprinted the page of the application that had the preference section and changed it. I also typed up a cover letter briefly explaining that "since turing in my application I had a visit opportunity and I am confidant that USMA is right for me, please accept this amended copy of page 8 etc..." The reason for this was so that they could have documentation of the change and I would not worry about some secretary changing it over the phone and so that it was clearly explained that I had put thought into my change of heart. Since I am close enough I drove down to my congressman's office and turned it in but mailing would have worked also. I would say definitely try to switch it because the worst case is that they say no, but if you dont try you may regret it later and Dont wait for your interview.
