Welcome!

Looks like doglover is right on this one. It's a bit confusing reading the fine print, but here it is.

Upgrade options are valid on most published fare types (and on many Northwest Airlines WorldVacations unpublished fare types) except for the following: frequent flyer program free/discounted award travel, VUSA passes, Visit USA fare, Round-The-World, Circle-The-Pacific, infant/child, Asian origin spouse fare, companion fares, Asian consolidator ID tickets, Denied Boarding Compensation, free, no value, agency/industry discount tickets, chart flight tickets, senior discount tickets, student fares, Fly Later-Fly Free vouchers, CATY International, most unpublished fares, consolidator tickets issued on 01298 ticket stock, any certificates, coupon, Bonus Miles or promotional offer/ticket, trans-Atlantic fares beginning with S/K/V/Q/H//M and U.S. point of sale trans-Pacific fares beginning with H/K/V/Q and H/K/V/Q/M for flights between the U.S. and Hong Kong/China. Northwest will determine the eligibility of new fare types as they become available.