I've found that sometimes, even when I'm told that E+ is "full", I can even get them to open them up for you just by getting the right agent. I'd recommend calling back every few days to see if you can get a better seat. You could also use trial bookings or something like ExpertFlyer (if you have a subscription) to check periodically to see if any seats open up.

Even as a 2P much of the time, it's been a looong time since I got stuck in E-. Sometimes it has taken a fair amount of persistence, but it's worth it to me to avoid hours in a cramped E- seat.

My recommendation: Keep trying to get the E+ seat and consider an earlier flight as an alternative. As another poster pointed out, you can check out the earlier flights at the airport for standby / upgrade possibilities without any risk, as you can always decide to keep your originally scheduled flight.