The best way to get an operational upgrade on CX is to have a booked seat and be sure your elite status (if you have it) is recorded on the manifest by giving your FF number when booking.

CX upgrades only when the rear cabin is oversold based on fare paid, as well as their own and other oneworld partner elite status. Selection of those being upgraded will generally be done well in advance of check-in, so rather than risk someone without a pre-selected seat being a no-show, they will move someone with a pre-assigned seat up to J-class first. Without a pre-assigned seat, you will likely end up in the seat of the person who was upgraded.

Have you checked the seating chart on ITN to see how full the rear cabin is? Otherwise, the chances of an upgrade are very slim to none, particularly if you are flying on a discounted ticket. That's been my experience, at least, a couple of times now on CX.