Question about Same Day Changes
 

I am familiar with most of the rules about same day changes - fees typically of $50-75 to get a confirmed seat and, with few exceptions, no free standbys. What I don't understand is this -

Example -
Airline has a 4 pm departure that is completely full, possibly even overbooked. Airline also has 3 pm flight with about 20 seats available.

Airlines will not allow anyone on the 4 pm flight to switch to the 3pm flight for free.

Would it not be in the best interest of the airline to allow such a switch? It makes people happy who want to go on the earlier flight and reduces or eliminates the possibility of denied boarding and denied boarding compensation on the later flight.

The only logic I can see on the airlines side is that this would lead to people booking the 4 pm to be sure of getting a flight but then trying to make the 3 pm. Which would be hard to plan for.

That is not true. Multiple times have they rebooked me for free on earlier flights if the confirmed one was heavily overbooked and they had room. And this even on airlines that do not have SDC policies. It usually helps a lot if you tell them that you know the later flight is heavily overbooked and you are willing to go earlier...