Originally Posted by Microwave

First, to clarify, there is no such thing as a "confirmed stand by purchase". There are two options for same-day changes for flights in the U.S., Puerto Rico, U.S. Virgin Islands, Canada and the Caribbean: first, you can pay for a confirmed flight change, as long as E inventory is available on your desired flight and your desired flight leaves in less than 12 hours; second, eligible passengers can stand by for earlier or later flights at no charge. The kicker is that, if you just happen to miss your flight completely, you'd actually be invoking the flat tire rule (FTR), which is different altogether. In such case, you'll typically be placed near the bottom the PALL list for the next flight (even if it's the next day) and be allowed to stand by even if you'd ordinarily be ineligible to do so. This should not require a reprice and change fee (as long as you really did just miss your flight).