I believe this is a length trial on upgrade procedure.
The reason is for each flights the upgradable spaces are limited and many EXP left unhappy if the upgrade request can not be fulfilled. So some better procedure should be produced as there are so many EXP around nowadays.
When a non-elite is selected to pay for upgrade, the non-elite would be interested how to get upgrade again and then they will learn flying more to be able to get free upgrade. Also AA would gather data on how much a person would pay for an upgrade. So for some non performing routes, AA could see more potentials of selling cash upgrades where not many elites on board. I do not think the cash upgrade would be extended to many routes as it has been existed for at least three years.
So I think for the elites croweded routes, the current procedure will remain. For some non elites routes, we may see more cash upgrade sales. I would not be surprised to see AA to adopt bidding system.
Personally I think no one is entitled to free upgrade unless for operational reason. Just personal opinion.