Just because you've never had success doesn't mean it's systematic. But also, why the assumption that F is full due to complementary upgrades? They do sell these seats, and they do have other mechanisms of upgrading (with net revenue impact). The assumption that F0 means that complementary upgrades were given T24 is simply not correct and would really be horrific revenue management (and we know AA are pretty good at this).
In terms of same day change, what one giveth, one taketh away ... so when one moves, it open up a seat as well. Also, and this is where there isn't transparency, just because a GDS says F0, it doesn't mean there are no seats available ...
And yes, it depends on the route and the season, but that applies to revenue earning too. There are times of year that they may sell more F seats than others.