Okay, I just looked at BOS-HKG (forgetting about sector marriage to PVG - no difference it seems) - 12/15 is W3R0 and 12/20 is zeroed out. Using the seat map (a good conservative estimate for F/J/PEY), 6 and 4 seats left for the two dates respectively.

Here's the plot twist, J class looks wide open - 22 and 21 taken for 15th and 20th. If it remains light load until close to departure, I could see them selling more PEY seats still and just roll them over into J class

I would say control would (might) take into account whether it is worth it to release your seat on the 20th and give you one on 15th in return. As for downgrade, generally CX is unwilling to work on 'downfares' (as opposed to 'upfares' - ie if you go for R class or I class on BOS-PVG) - these are embedded in the fare rules.