In the past there was a glitch in the codes CO sent to NW so that, if you got upgraded, CO sent NW the class paid and flown as R (the upgrade code back then). You'd then get 100% EQMs. But that was fixed long ago. CO does a pretty good job of preserving the class paid in your record through upgrades and re-routes.

Your only real chances of avoiding the 50% EQM are (1) buy a higher fare class on CO, (2) book via NW as a NW code share and with a NW fare class higher than V (some of the CO 50% fare classes translate into Q or better on NW), or (3) hope for an irrop and get rebooked on NW or DL. If the latter, you usually get rebooked in full Y and get 150% EQM.