American Airlines will let you confirm an upgrade at booking or any time before travel using miles (plus a cash co-pay) or systemwide upgrades (which are a choice benefit for Platinum Pro elite members and above, and given out to million milers upon qualification).
However the airline is super stingy with this space, even on flights that are completely empty and likely to remain so. But a glitch is giving away upgrades to customers that American Airlines didn’t intend to offer.