When I log into my reservations on the app I'm often prompted with a message to upgrade to F for some amount of money. Typically the amounts aren't worth it (last week $1,041 for an upgrade on MIA/LAX 321), at least to me. But maybe others bite. I assume this would be an instant upgrade after one paid the $$$$.
I've noticed that after AA instituted this feature in the app my upgrade rate has plummeted. AA seems willing to sell upgrades via the aa,com app. IDK if AA would sell the same upgrade to a non elite logging into their reservation.