This mileage upgrade program seems to be much more restrictive than what everyone is posting. I think the disconnect may be that some are posting what their own frequent flyer program offers as upgradeable fares, whereas the Skyteam Mileage upgrade program seems to be truly FULL fare economy.
NW offers its own frequent fliers upgrades with Y/B fares. Delta Y/B/M fares, KLM Y/S/B and AF Y/S/B/K. However as an example, when I look at the following NW chart, NW fliers can only upgrade AF Y fares, not Y/S/B/K like Flying Blue members:
Skyteam parner upgrades for NW:
SkyTeam Carrier Marketing and Operating Code Eligible Full Fare Booking Classes:
Aeroflot SU S, Y
Aeromexico AM Y
Air France AF Y, S
China Southern CZ Y
CSA Czech Airlines OK Y, M
Delta DL Y
Korean Air KE W, Y
Alas, at these fare levels it is usually cheaper to buy a discounted business class fare than use the miles to upgrade. Then again, it might be a advantageous for last minute travel.