I'm not sure this is correct. On the contrary, J-F op-up in my experience is pretty common. As all op-ups are, I suppose it's route specific - but I have received an abundance of them in the last 12 months. Since March of this year, I've done 12 long-haul (HKG-USA or USA-HKG) segments where I was originally booked in J class. Out of those 12, I have received 5 op-ups to F class. And on one of the times where I didn't get it, op-up was on the way......but then the plane went tech and many people on the flight opted just to go back to a hotel and not wait in the airport for when we finally departed 5 hours later. Hence I was relegated back to J.

Actually, J-F op-ups are one of the main reasons I'd argue to KEEP flying CX as a DM. My anecdotal experience seems to indicate that they happen more often on SFO and LAX flights. I have also received it flying to YYZ. (I have never once received a J-F op-up on HKG-JFK). But the possibility of them definitely keep me loyal to CX when I book J class tix. I have thought about this before, and I think if CX were to take away the op-up possibilities that would probably change my calculus and make me start booking more *A tix. But for the time being, with the rate of op-ups I'm hitting, I find it a great benefit.

When booked in J, there are two ways (instead of just one for EY) that you can get a J-F op-up. The first is if the J cabin is oversold. However, the second is if EY is overbooked by so much that surplus goes into J, where that surplus (aka, DMs originally booked in J) then gets bumped to F. On my SFO-HKG op-ups, this is almost ALWAYS how it happens. There are a few empty J seats, but Y is oversold by 10+, and then they push as many people from J-F as possible in order to make room for EY op-ups.