Standby (and pay up) for take off

Q. On a recent trip from Philadelphia to Pittsburgh, my husband and I wanted to try to fly on an earlier flight. When we approached the US Airways ticket counter, after landing from our flight from Lisbon, we inquired if there was space on an earlier flight and might we be put on a standby status.

The ticket agent explained that because there were seats available for the flight and it was not oversold, she could not list us as standby. We would each have to pay a $50 change fee if we wanted to fly on the earlier flight. Standby could only be available on over sold flights.
Can you tell me when standby no longer became standby?

A. Every airline has slightly different policies for same day standby, and these change from time to time.

Here's US Airways' policy from their web site:

Move up program

We allow you to 'move up' to any earlier flight on the same day of your originally scheduled departure time at the airport (with the exception of flights to Hawaii and Europe). You can only make day-of-departure changes at the airport (and not by calling Reservations).

If there is an open seat available on any earlier US Airways operated flight that departs on the same day as your originally scheduled departure, you may change to that flight and we will automatically confirm your reservation for $50 for flights within the 48 contiguous United States and $50 for flights to Latin America, the Caribbean, Canada and Alaska. This lower confirmation fee replaces the usual $150 change fee, plus any differences between your old fare and new fare, for non-refundable tickets.

If there is not an open seat on an earlier flight that departs on the same day as your originally scheduled departure, you may stand by for that flight at no charge. Standing by at no charge is not permitted if we can confirm that there is an open seat available for that flight. Seats on completely booked flights may become available if passengers with confirmed reservations don't show up for the flight. Unlike a confirmed reservation, flying standby does not guarantee that you will get on the particular flight that you want and may involve waiting for two or more flights before a seat is available to you.

On JetBlue, by contrast, you can go standby for the next earlier flight at no charge if seats are available. If you want to fly on the flight before that (assuming there is one), there's a $40 same day confirmed flight change fee.