I think you have your terminology mixed up. What you talk about is referred to as a 'phantom UG'. An op-up, by definition, is an operational upgrade due to a lower class being oversold (e.g. Y is oversold and some pax get UGed to J, J is oversold and some pax get UGed to F, where that's avalaible, such as AF). Different airlines have different policies for op-ups. I know one airline that UG's the last Y pax that check check in (if you check in and there's no room left in Y, they'll UG you to C). Other airlines do op-ups based on elite status and/or fare class, others have some other indicators, such as million miler status, some secret high-revenue pax index, etc.

I don't know for sure how NW does it. Although I did get a phantom UG once, I never got an op-up on NW. I got several op-ups on KL as a WP Plat. I always watch the availability of the flights I'm about to take and all the flights I got op-uped on were oversold in Y.