CX seems to have a policy of overselling J and then bumping premium pax to F as required. This seems to be different than SQ for example, who requires purchase of a F ticket should J become booked.

Choosing flights carefully helps your chances. My best experiences have been on LHR-HKG and HKG-SFO on Fridays. I'm on a waitlist for HKG-SFO in J later this month. It will likely clear, and the chances of getting an upgrade are high for a OW Emerald. I've been in the Wing before where they've called at least 10 people (to pick up their upgraded boarding passes) on flights from HKG to NA.