Plans can change for any number of reasons. I don't think USNA would think less of an applicant who needs to modify their visit date. Visiting ANY college your are serious about applying to should be part of your process and consider it an investment in your future. Some visits may cost more then others, but deciding to join the military and the associated commitments is a BIG descision. There have been various threads in the past about the topic of CVW evaluations. I don't know if they do/don't evaluate candidates, however I would always behave as though they ARE since you only get one chance to make a first impression.