I have continued to look into this issue. It turns out there's a DOT regulation specifically on point:
§ 253.9 Retroactive changes to contracts of carriage.
An air carrier may not retroactively apply to persons who have already bought a ticket any material amendment to its contract of carriage that has significant negative implications for consumers.
It seems AA has been taking the position that it is permitted to modify the COC and hold passengers to the revision. DOT regulations here indicate exactly the opposite.
I'd be interested in hearing the experience of affected passengers. I know at least one passenger who was denied transport despite complying with the COC check-in time requirement posted
as of her date of travel. Another was denied transport despite complying with the check-tine requirement posted
as of his date of ticket purchase although AA purported to have changed that requirement in the interim (exactly what 253.9 disallows). Anyone else?