well, for KA flights, at least HKG-PEK and HKG-PVG v.v., it's definitely the J cabin that's filled up first well before Y, esp the Sun PM / Mon AM / Fri PM flights...I'm flying HKG-PEK v.v. weekly too and I, even as a CX DM, have problems getting myself booked in full-fare J on my preferred flight every single week too...it was better a few months ago when KA Elite was separate from CX MPO, and I was KA Plat...now it just seems like there's so many more CX DMs competing with me on waitlists, and at the same time CX is probably doing a very good job feeding all their long-haul connnecting pax on KA flights...now the situation is quite a bit worse, at least from my perception. Basically I book all my flights 3 months in advance, but since I often need to change my flight in the last minute, basically I'm on waitlist for J every single week...quite annoying really.

Well - to make it short, my point is - there's really almost 0 chance getting op-up Y to J on the HKG-PEK/PVG v.v. routes (except flights like Sat morning that have much more tourists than business people) - there's so many people on J waitlist that they can easily take them (and the $) instead of op-up.