Originally Posted by RobOnLI

Directly from the page:
Northwest prioritizes upgrades based on the type of ticket purchased (full-fare vs. discount fare), Elite status, class of service (within Elite levels), date and time of booking. Full fare tickets include Y fares or -26/-27 type fares. Discount fare tickets include all other fares.

It clearly states right in there that class of service (fare class) is absolutely part of the equation.

-RM