Originally Posted by newyorkgeorge

No one really knows the specific algorithms AA uses for releasing upgrades. Seats may be open but AA might believe (whether correctly is another matter entirely) it can sell those seats. Second when Y is sold out or oversold AA might be able to get flyers to pay for F in order to have a confirmed seat assignment. Third, AA might want to keep open F seats in the case of misconnects for paid F paxs. Generally AA holds back one or two seats until departure time and assigns those upgrades based upon the upgrade order (again that's not always 100% practice).