AA has what they call the PALL list that controls the order of upgrade and standby requests for the gate agent. In the OP's situation he would in theory be classified as an involuntarily downgraded premium ticket and would be at or near the top of the queue in the event a J seat is available. But I believe this only works at the gate and you may need to get an agent to put you on the list appropriately. Otherwise, if you accept a downgrade you're stuck from that point. You would think they could automate something, because they always preserve the class of the underlying ticket for the purpose of FF credit, regardless of class flown.

It's a similar deal with award ticketing, if one segment on a J award is only available in Y but subsequently opens up in J you really have to fight to get them to fix it, especially if the J award is no longer available or has become more expensive on the longer segment.