I think the OP was talking about a free upgrade to E+. If E- is booked solid , they will have to move some passengers up to E+. Just dont select a seat assignment, and they'll give you one at the gate. If E- is full, you'll get a seat in E+. However, waiting till the gate to get a seat assignment puts you at risk for being separated and being placed in two separate E- middle seats.
FWIW, I wouldnt spend the $200 on E+ upgrades. I have done ORD - HKG - SIN in E- middle and lived.
If you have the cash and are a tall person, you'll benefit from it though.