I just received a fantastic double upgrade from Y to J from HKG to LAX but no idea why / how. I booked a S class fare a few months ago, called to try and either pay to change into PEY or V class (so I could at least earn club miles) but both PEY and higher fare classes were sold out. Today, I get an upgrade at check-in to PEY (with thanks!) and then got bumped to J at boarding. Our equipment had a malfunction identified at the gate so we need to switch planes so now I'm crossing my fingers that I don't suddenly get downgraded back to Y.