CX/KA do "upgrade" selected passangers in J to the first class seat but still provide J service. That's why expertflyer shows these are J cabin.
Besides, only station supervisior where the flight departs has authorization to determine WHO and WHEN to get this gesture.
Call center or MPC agents can not process this offer.
WHO: DM for sure, GO/OWE/SL would possibly get this too.
WHEN: It could happen few days in advance, or at checkin counter, or at boarding gate. It depends on the loading and checkin stiuation.
My suggestion is to just ask for it at checkin counter. There might be a surprise.