Well, regardless of your status, it's very unlikely that you'd be able to upgrade from Y to F on those flights. They're three class planes, and United only allows upgrades from Y to C or C to F these days. ("No double upgrades.") So, you could buy a C class ticket and upgrade it, but you'd probably be paying almost as much as you would for full fare first. The transcontinental F service is nice, though; you get about as much room as you do in international F.

Of course, there's always a possibility that you might get upgraded to F due to agent error or overbooking. This has only happened to be once, though, and I'm often on three class domestic flights.

-Adam