You can fly standby for free on the immediately previous flight to the one you have currently booked. So if you are on a 11am JFK-AUA and there is a 9am that you want to fly on you can do that as a standby for free. You would do so at the airport.
If you're on a 7pm JFK-LAX you cannot switch to a 9am (rather than a 3pm) without paying a change fee.