It really depends on the station, and the loads. From my experience on so-called "firm J" (13J42) tickets, I've been upgraded to F only once and that was due to personal connections. I've been downgraded often, and even left behind several times. BA
has a reputation for being really tough on upgrades. Unless your nonrev ticket category is specifically upgradeable, it is more than likely that they would leave you behind on a full flight rather than upgrade you. Best to get to know someone at PHX
station. There is some discretion, but only a little.
It's been swell, but the swelling has gone down.