I'm pretty sure CO will pull any upgrade you get on CO flights using your NW FF # unless you also have elite status with CO. Might be academic though, as last year I got upgraded on CO flights as CO gold so little that I switched this year to NW. If you fly NW I believe you can switch to CO FF # without losing the upgrade, but if you fly lower fares on NW, you only get 50% EQM with CO which will turn NW gold into CO silver for the same amount of flying.