US does hold some back, but at 30 hours out all of F seems to dump into "O". And FWIW they do appear to upgrade by status level even as new "O" inventory is added, but quite honestly they should hold a few seats back for the gate to process. At 2 days out most flights show F0 and while I can see the arguments that an SP deserves an upgrade it isn't right when a CP has to make a change or buy a last minute ticket and winds up in F. The old US (East) always held seats until the gate (often too many seats).