i've tried upgrading KLM flights on numerous occasions and usually what gets me to quit is that it seems 90% of elite reps simply dont know how to do it....i've spent hours on the phone trying to hold their hand reading through intructional guides they have as well as fare rules, yet in the end they still dont know how to do it...

marshall200 is correct though, you will need a B or higher fare to upgrade, but on the plus side you will not have to pay any BF upgrade fees as you are used to with CO

most likely your flight is a NW/KL codeshare, may i suggest in the future trying to book a NW ticket (B class or higher again)and upgrading from there, i feel CO reps are more comfortable with the NW rules and processes