It used to be that you could standby for an earlier or later flight on the same day of your original flight without paying a change fee.
These fees are just another way for airlines to make money, and have replaced the prior free standby policies. But, on the plus side, at least you do get a confirmed seat versus a standby. Here are the official policies of some U.S. carriers. Note that these policies apply to domestic flights and may be different for international travel.
This is the only airline we found that doesn't charge a same day change fee. "Customers may standby at no charge for flights other than the one for which they have a reservation on the same day of departure," AirTran's official policy says, plain and simple.
"You can now get a confirmed seat for same-day flight changes for domestic travel on all fares for a $25 fee. Passengers can call Reservations or handle the transaction at the airport ticket counter or self-service check-in machines. If eligible seats are available within 12 hours of departure of alternate flights for your same itinerary, your flight change can be confirmed. American continues to offer the same day option of standing by for an earlier or later domestic flight for eligible fares at no charge."
"You can now get a confirmed seat when changing your flight for travel on the same day for a fee of $25. This new convenience means you no longer have to wait as a stand-by customer if there is an available seat on an alternate flight."
Delta Air Lines
"The same-day confirmed option allows you to change your flight time on the same day of travel for a fee of $50. To use the same-day confirmed option, you must confirm your new flight within three hours of the scheduled departure time. If you have a flexible fare (usually a refundable ticket), you may be able to change your itinerary without a fee."
"For travel to the same destination, reservations can be changed to standby travel only on the flight immediately prior to the original departure without an additional charge. Standby travel is not available in markets that only have one flight a day or if there are no flights prior to the original departure. Confirmed seats are available for $25 on an earlier or later flight on their same day of departure."
"Northwest Airlines … expanded its popular FlyNow option, which permits confirmed flight changes within three hours of scheduled travel for a $25 fee, to now include flights throughout the entire day of departure."
Fee: None, but you may end up paying more than on any other airline
This airline is a whole different animal. While most customers love the fact that if you change your travel dates on a non-refundable fare and decide to travel within a year from the original flight (even on a different route), you'll be able to do so without a fee, it's a whole other situation if you want to fly same day on a different flight.
Although Southwest doesn’t state this anywhere on its site (that we can see, anyway), if you're on, say, a $99 one-way fare leaving at 5 p.m. and you want to leave now on the 2 PM flight, while there's no "fee" as such, you will have to pay the difference between your $99 fare and whatever the current "walk up" (no advance purchase) fare is on that route, which could be hundreds more. As with other airlines, Southwest sometimes looks the other way, but if the gate agent isn't in a generous mood that day, prepare to pay.
"You can confirm a seat on an alternate flight on the same day of travel anywhere United flies within the United States and within four hours of the time of your request. If eligible seats are available on an alternate flight, your change will be confirmed and you will be charged a $50.00 fee."
"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 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 $25 for flights within the 48 contiguous United States and $50 for flights to Latin America, the Caribbean, Canada and Alaska.
”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."