Even for Diamond MPC members, it is not a given that we will be upgraded although if a flight is showing zero in Y and J, the chances are good.

I've been left in economy when I purchased an economy ticket at times, and yet, the lucky ones, when I've been upgraded on the long hauls like YVR-HKG-YVR on a coach ticket, I thank CX profusely.

I've never experienced a double upgrade .. yet although they do happen very rarely. It's also nice to know CX ground staff (in asia) personally because they can override your priority in the UG ladder.