Originally Posted by GoBob

Just a note that for same day confirm, fare code/class is the measurement, not cabin. So if you booked an X fare in Main then you need an X fare available on the flight you wish to change to or you'll be charged the fare difference. The change fee is waived but the fare difference can be substantial (speaking from experience as a Gold/Plat). Standby remains free but it is of course a roll of the dice. Calling in, an agent can usually give you a sense of whether you'll be successful with standby.