I attended McGovern's preview day, and though some were dressed as if it were an interview, most dressed down at last somewhat. I wore my suit pants, nice dress shoes, and a patterned button-down shirt with no tie. Basically, I lost the jacket and tie and swapped the classic white/blue suit shirt for one with a more casual look.
When in doubt, it cannot hurt to dress up a bit, but I wouldn't stress it. Many people wore khakis and polos at the McGovern preview day, and the tour guides were in sweats/jeans and school polos. I know this isn't TTUHSC preview day info, but I hope it helps some.