Granted you were lucky and I think the agent did the best move for both you and the airline. If I was a betting man, I bet she was looking at your morning flight and seeing a way to lower the oversell situation on the morning flight. Even if the flight was not oversold, it still provided goodwill for the airline and the customer which will probably bring them more revenue in the future than the one time change fee. Be sure to take that agent a little gift on your next flight!