the sold (or oversold) condition of the flight should have no bearing on the ability to change or stand-by. usair's policy is earlier stand-by same-day only, with perhaps a $25 charge. later flights are considered a change, thus the $ quoted you.
an oversold condition could afford you a double bonus, bumping you to a later flight, and paying you off with a voucher. of course, you'd have to be at the airport for your scheduled flight -- which is what you are trying to avoid in the first place.