We had the exact same thing happen for our Nov 11th flight on United. I'm not sure if this is an option for you, but we had United put us in the 7:30pm flight the NIGHT BEFORE with no change fees. We take our neice(4) and nephew(8) and would rather have them tired and cranky on the late night flight and then get a good night rest and wake up raring to go. United did the exact same thing last year on these flights and we took the early flight with them and the niece had a pretty bad first day. This option does cost one more night of hotel (in our case DVC points), but an extra night at WDW is never really a tough sell for us ;-)
You could always get a full refund and rebook the American Airlines flight out of O'Hare. They have an 11am on your original date and it's only $76 one way. Either way you guys will have a great time!!