I seem to recall "changing the number" doesn't do the trick - somehow the original FFP details sticks in the PNR. You have to "remove" the number entirely. I forgot what these fields are called in the industry technical terms.

I would say if this disappear from Manage my booking on both CX.com and partnerFFP.com, i think that's now set. Obviously you loose the op ups chances (do they still happen?) or recognition if flying in economy but to strategically allocate the credits maybe worthwhile.

For me personally when I do purchase, I credit them to Qantas as the Club Sector equivalent (Status credit they call it at QF) for CX on non-competing routes for Premium class is reasonable, even points credit mediocre. (HK-Aus are different story, QF competes so you get almost nothing) I gave up trying to fiddle around removing my FFP at gate/lounge etc to game the system, I rather have a comfortable and smooth experience than macking about.