It's highly unlikely that AA would involuntarily downgrade anyone who already has a seat assignment. The only normal exception is someone who was upgraded at the airport. There are also anecdotes of an agent picking out the most recently upgraded passenger and approaching them with an offer to downgrade, but it is normally some kind of VIP that needs/wants the seat.
Ok well one time my MIA JFK flight was super delayed and I barely made it to my JFK SFO flight only to discover that my seat had been given away to a nonrev. I put my foot down telling the gate agent I had still arrived on time before she closed the flight and though I was given all sorts of bad looks, I got my F seat back and the nonrev was sent back to Y. I don't like calling myself a VIP but my seat shouldn't have been given away when it wasn't even my fault that the flight from MIA was so late.
