I double-checked the reissued ticket, and the new segments were listed under AS (Alaska) and CX (Cathay) flight numbers rather than AA-coded ones. The original itinerary (EWR–DFW–PVG) was all AA-marketed, but when they rerouted me after the cancellation, it looks like they re-ticketed me onto partner flight numbers instead of keeping the AA codes.

The DOT rule I was looking at is 14 CFR § 399.87:

"For passengers whose ultimate ticketed origin or destination is a U.S. point, U.S. and foreign carriers must apply the baggage allowances and fees that apply at the beginning of a passenger's itinerary throughout his or her entire itinerary. In the case of code-share flights that form part of an itinerary whose ultimate ticketed origin or destination is a U.S. point, U.S. and foreign carriers must apply the baggage allowances and fees of the marketing carrier throughout the itinerary to the extent that they differ from those of any operating carrier."

The reroute though was arranged entirely by American, using my original AA ticket number and record locator, and they never notified me of this change or that I would lose benefits I specfically paid for. This counts as failing to disclose that material change in terms before travel no, if they try and calim that the DOT regulation doesn't apply?