I have always confirmed same day changes for co-terminal on that day.

In other words, I turn up at the airport and say I'd like to go to DCA instead of BWI, and they say "Okay, Mr. Gleff, here's your boarding pass."

It's not really standby -- but I haven't ever done it successfully over the phone in advance of the day of departure.

Back in December, I was flying IAH-ORD-BWI and I called up the day before and asked about switching to IAH-ORD-DCA and was told no go. No problem at the airport -- nor have I ever had such a problem.

Since it was a ticket w/o Sat. stay or much advance purchase, there was about a $1k difference in fare between BWI and DCA. So, I ticketed BWI and then left out of BWI and returned to DCA in order to save the $1k.