Originally Posted by
stam
It sounds like you are asking about two different things.
Getting upgraded to F on a CPU is pretty difficult for gold and silvers unless you're flying at off-peak times and on semi-unpopular non-hub routes.
As a gold, you can pick an E+ seat when you book the flight. As a silver, you have to wait until T-24. There usually are E+ seats still available at T-24, but if you check in late, you might get stuck with a middle E+ seat. Just keep an eye on it throughout the day before your flight as a silver. You can always change seats after you checked in.
You can reselect your seats, but be aware that in order to do it, you have to go through the check-in process again. You can't use the usual method of changing seats. Still, I've re-checked in a number of times when I've seen a prime seat (like exit row aisle) show up when others get their F upgrades).
I've rarely had to settle for an E- seat, even when it appears full. I just wait for the upgrades to take effect and snatch up the seats when they show up.
I must admit that I set an alarm for T-24 to remind myself to check in as soon as possible so that I can get an E+ seat.