Originally Posted by JJeffrey

It's my experience that until the new flight completely closes out, you're still on the old flight as well.

Another example...LGA-ORD last Fall during the pilot/mechanical fiasco. I clear standby on a flight an hour earlier. Take my seat, but at departure time the pilot comes on with a problem. After 20 mins, they start letting people off the plane. I run to the AC, where they told me I hadn't been taken off my original flight yet. Even cleared the upgrade at the gate for my original flight.