I once changed an American flight where I had a cheap 21-day advance one-way fare; the original fare was something like $49 and I wanted to change to an earlier flight. They wanted $75 to do it even though I could have purchased a 7-day advance fare then for something like $63 so I told the reservationist she was nuts as it'd cost me less to buy the new ticket. They made the change and were going to charge me around $50 instead after I talked to a supervisor; but they never charged me the $50 so it turned out to be a free change..