It varies greatly. The more popular a show is the less flexibility the box office has. They aren't under any obligation to refund your money. It's not their fault either!
That being said, I've always had good luck what few times I've sought to change tix I've already purchased.
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.