Load factor based upgrades (LFBU) do still exist. AFAIK, they pull from the same instrument upgrade inventory bucket (A/C) and aren't offered until all requested/wait listed upgrades clear. Then, if there are still seats, revenue management MIGHT offer some for sale. I'd recommend checking actual upgrade inventory on expertflyer.
Also, for your reading pleasure:
https://www.aa.com/i18n/fragments/merchandising/upgrades/LFBUTerms.jsp
http://www.flyertalk.com/forum/american-airlines-aadvantage-pre-consolidation/1702228-does-aa-offer-day-departure-paid-upgrades-international-j.html
http://www.flyertalk.com/forum/american-airlines-aadvantage-pre-consolidation/1676716-aa-new-cash-upgrade-offer-reservation-details-page.html