To be honest, I use Expedia for almost all my travel, business and professional, and I've never had a problem.

In fact, they helped me out a great deal in March, when US canceled a flight I had booked to MEX for a business trip and would only offer to reaccommodate me on another US flight the following day. After I talked to my boss and we made the decision to buy a new ticket for later that day, I couldn't get through to US to cancel the US-rebooked flight because their phone lines were swamped - but Expedia took control of that and canceled for me. That definitely saved me lots of time and hassle because otherwise I would have had to fight with US after the fact.

I'd ask whether the OP received an email or other confirmation from either Expedia or AA showing s/he was ticketed on the new flight. It sounds like that didn't happen. OP, if you're around, what form did the initial contact from Expedia about the flight change take?

Also, fear of stuff like this is why I always print out a hard copy of the ticket/itinerary before I travel. And ALWAYS reconfirm the reservation before going to the airport!

At any rate, I've had pretty big problems in the past with tickets being assigned from one airline to another - even without a TA's involvement - so I'll personally refrain from fleeing Expedia at the moment.