Note that if you do it online with website, you must do it through the Check-In link and then select Change Flight option. You can continue to use Check-In link on website even if you have already checked in. Unfortunately, it only lets you SDC to flights within 24 hours of current time. So you if you want to SDC to a flight later in the day than your original flight, you need to wait until T-24 to new flight (and it's possible availability may disappear while you are waiting).