Why would BOS route not have E class (or, PEY)?

Question would be what class (I suspect H or I) is OP booked into for HKG-PVG.

Anyway, don't worry, my experience is no matter to however yield-trashing outport, CX will clear revenue waitlists before releasing award seats. I've done it a few times as OW EM and it never took more than a week (more like 4-5 days) to clear. So if OP has status, eventually it should be ok. Don't forget you can WL for 3 dates.

Anyway here's whats on sale now: BOS-HKG-PVG, POS US

CX 811 0 BOS
12/15/17 1:30 AM HKG
12/16/17 5:55 AM 77W
M,F,Sa
92% / 45m F5 A5 J9 C9 D9 I1 W3 R0 E0 Y9 B9 H9 K7 M6 L0 V0 S0 N0 Q0 O0

this can jump up and down every week so you just have to keep an eye out. If the flight is empty (and not like HKG-PVG should be worrying), it'll gradually go up closer to departure.