I don't think it makes any difference if you are CO or NW elite and are trying to upgrade a coach ticket to Asia.
A ticket booked in V class, the cheapest ticket available in the ORD-BKK market and currently $863.80, IS upgradable, however there have to be D seats available, which is very difficult to find. This is a change (back in Aug, I think) from previous policy where there only had to be C seats available.
You can upgrade a full Y fare ticket into WBC from the C bucket, and this is what the agent might have been trying to say.
Again, I really don't think CO elites have an advantage over NW elites when trying to upgrade on NW metal.