This is their policy on changing to an earlier flight; from their website:
We allow you to move up to an earlier flight on the day of departure. You must be at the airport to MoveUp. The MoveUp fee is $75 for flights within the United States, Latin America, the Caribbean and Canada. It's $150 to move up on flights to/from Europe, Israel and/or South America.
You can move up for $75/$150 if the flight is:
- Within 6 hours of your original scheduled departure
- On the same day with the same number of stops
- All flights have open seats (if you have connecting flights)
We waive the MoveUp fee for Dividend Miles Preferred members. However, the same rules apply: the flight must be within 6 hours of your original scheduled departure, and on the same day with the same number of stops.
Standby
Seats on completely booked flights may become available if passengers with confirmed reservations don't show up for the flight.
If there isn't an open seat on an earlier flight that departs within 6 hours of your originally scheduled departure, you may stand by for that flight at no charge.
You cannot stand by at no charge if:
- We can confirm that there's an open seat available for that flight
- You have one or more connecting flights