I don't think it matters at all. I routinely fly KA and CX interchangeably on each others' flight codes (to both PVG and PEK, where they both operate), and get op-ups under each situation. Boarding pass can also be issued on both KA and CX stock. As poster above mentioned, they just print out a line on your BP that says something akin to "booked as CX6991" or whatever, adding the extra digit to the front of the otherwise CX or KA flight number.