From FlyerGuide Wiki

For the most part, an award ticket is no different than any other ticket, ONCE IT IS ISSUED. Once the award ticket is issued, you can certainly stand-by and/or pay for a confirmed earlier departure on the same day.

However, the key here is that the ticket must be issued. You must have a valid award ticket issued for the day of travel. You cannot try to fly stand-by on a flight where no award seats were deemed available.