Sorry, didn't mean to confuse. What I meant is that if one is to try and buy a J ticket and J is booked, then CX will often sell a few more J tix and bump the premium flyers from J to F as required before the flight, while some others seem to require purchase of a ticket in a cabin that is not full.

While that's what I meant, this is solely based on experience and not inside knowledge of any airline policy, and, from other posts here, is possibly inaccurate. Guy Betsy, who would know better than me, points out that my experiences have likely been the result of some Y passengers being bumped from Y to J and then me and others from J to F. My assumption was based on being able to get a J ticket when the seatmap shows J full, and then getting bumped to F (I know, don't trust seatmaps). But with CX, when flying J, if the seatmap shows J as full then I believe I've been about 100% succesful with op-ups from J to F.