Did you buy the tickets thru Telecharge online or over the phone? If so you'll have to return/exchange them by calling them. Telecharge has always accommodated me when I could not make a show, sometimes they will have to talk to a supervisor or the box office, but they always return calls if they do. I am not sure how accommodating the Box Office will be though, they may tell you that you will have to past date your unused tickets. Good luck