It’s one thing to get a discount fare on an airline.
But once you book it, don’t try to change the date or the time or you could pay three times as much with the requirement to pay the difference in fares.
If you book a ticket between San Francisco and Washington D.C. 21 days in advance, you might pay $194. Book the same flight 20 days out and the lowest fare is $405 one-way.
The same applies if you change airports in the same city. Huge fare differentials can happen if you change from JFK to LaGuardia, even if the planes are flying half-full. The same applies between Washington Dulles and Reagan airports.
So, unless it’s an emergency, always keep your originally ticketed airport.