I've had great luck changing my connecting flight to an earlier one with American a few times. The reason I didn't book the one I wanted was because it cost a lot more. I've never been able to have them officially chage it pior to departure day, but have been able to ask the agent when I check my bags to be put on standby and they do and then I hurry to the connecting gate of the flight I want and ask again and three times (each time I've asked) over the last 2 yrs. they've been able to accommodate meband get my checked bag on the right flight. Always worth a try, but I don't think theyll officially change your flight until day of.