Originally Posted by erik123
I think for NW it is I when upgrading from a B fare (trans-atlantic) and Z when upgrading from Y.
This is the common viewpoint, but worth noting that some B-fares actually upgrade from Z and some don't upgrade at all.
Fully changeable and refundable B-fares usually upgrade out of the Z bucket and only require 10k miles as opposed to 20k miles each way. Example are the B2US and B5GB fares between US and UK.