There is no such thing as a sameday standby anymore. It use to be that you could standby for flights on the same day you were already booked. If you weren't already booked on a flight, you couldn't standby. These days, if you are booked on a flight... you can do a same day change at the airport for a flat fee depending on your fare level. This is often less expensive than a change fee plus difference in fare. The previous poster mentioned AC 405 at 122 +taxes. That may be the best thing. Hope this helps!