You can try it online or in App before calling in. See same-day change option in App or go through Check-in link on website and select Change Flights option. Although they won't show options until 24 hours before your later desired flight (calling in will let you select 24 hours before original flight). Same-day confirm may be an option instead of stand-by if there is original booking class inventory open on the later flight (assuming you are on a Main fare).