I think there's a good chance you'll get put back in E- after a sweep, since you're not elite. So it becomes a choice between hoping to hold onto your seat in E+ but then risking an E- middle seat. There's also a possibility you could get E+ on the day of the flight if you didn't have a seat assignment at all if E- is full. Pretty risky, though. If you really want E+, think about buying the annual pass or hope you are offered a buy up at check-in.