maybe you guys are right on that Expedia is indeed a discount retailer, but the flexibility there is same as buying it directly from AA.com. If i am willing to give up the flexibility, i would instead bid it from priceline. I got it for $221, and it's not really a bargain price, the price was like that at that time, and aa.com was selling if for $225. The reason why i bough it from Expedia is not because it's cheaper by several bulks when compare with other agencies, it's also because i have been satisfied with their customer services.
Anyway la, thanks all for your help. I complained to AA and they now agreed to reassign me a new flight that bears the same boarding time with my old flight.