There is same-day flexibility - advertised for A-list members but sometimes you could be accomodated anyway, or pay the fare difference. Day-ahead you have to change your ticket, for one thing depending if you were slightly more than a day early you won't be able to get a boarding pass, so it's not like you can wander on down to the gate and see how it goes. I don't know if TSA will let you through with a boarding pass for the next day...this might vary by airport, a 24-hour facility might not care as much as regional airports.
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