i was extremely fortunate (and grateful of course) that I got upgraded 3x on PVG-HKG-SFO and JFK-HKG-PVG itinerary (i flew AC between SFO and JFK with a transit in YYZ). HKG-SFO was from premium eco to J, JFK-HKG was from premium eco to J, and HKG-PVG was from eco to J
in conclusion, I think my experience is more an outlier / luck than a general practice when flying CX