DOT could not help here if it wanted to (it has never got into the middle of award messes). CX is not a US carrier and neither HKG nor MXP are in the US. OP could try the HKG consumer authorities for the good it will do.
To be clear, this has nothing to do with awards, it could happen on a cash-paid ticket, it is simply much less likely.
Regretably it happens with some frequency, especially with F tickets on carriers which do poor IM/RM and have "seller's remorse".
While AA has no obligation to do anything for OP, if OP does wind up taking the J seats at the new price, I would still go back after the trip is complete and ask that the matter be reviewed and that AA as a goodwill gesture, credit the difference to the old J price. OP would still need to be prepared for a "no", but, if framed correctly, it is fair request to make.