Same-day standby means you can take a later flight as well as earlier. If you can't get on, assuming the usual voluntary change rules apply, you will have to make a confirmed change to leave the following day.
If this happens on the first flight of your trip, your itinerary will also have to be re-fared up to the last-minute fare. The $100 change fee pales in comparison to a walk-up fare for most markets.
If you are doing this on any flight but the first, your itinerary does not have to be re-fared as long as you don't change your destination(s). You still have to pay the change fee.
[This message has been edited by JS (edited 10-12-2001).]