There have definitely been reports of this over the years.
The difference is that you can clear a 500-mile upgrade request in advance without having the stickers in your account. If you then refuse to purchase the stickers, and your original fare class isn't available, it kind of puts AA between a rock and a hard place. Either they force the downgrade, or they IDB you. I believe the former requires either a supervisor or a call to revenue management, but 99% sure that's what they'll do.

AA downgrades people from time to time, it's probably fair to say that not all those times is the original booking class available, yet they make it happen. It's just a matter of whether they want to or not...