Classes for upgrades are not visible. U (nor Z) are not the classes that flight upgrades book into - this is a common bit of misinformation floating around.

You can use them as an indicator, but the buckets are different, and not visible. Normally, if there is Z availability, there will be upgrade availability, but not always. There is no way to see for yourself online if upgrade inventory is available. The only way is to call FB. If you're seeing plenty of seats open in WBC, chances are good they will release upgrade inventory, so keep trying.

FYI - upgrade inventory on NW is very, very limited for FB.