As long as there aren't any int'l segments in your itinerary, you can do SDC - Same Day Confirmed - to another routing, as long as there is at least 1 seat available, and the new routing is considered to be a valid routing for the fare you purchased, but SDC can be a little complicated, as you can only attempt it as early as 3 hours prior to the new itinerary you want to take - AND, it's likely to goof up any upgrades you may have already gotten in advance. If you tell us a more specifics about your itinerary, perhaps someone here can help you a little better!