V-Flyer

When booking an upgradable fare on the VS site it gives S and W for PE which I understand.
It also gives Upper Class D fare option. If you are already in UC why would you need an upgradable fare ?

It isn't classed as an upgradable fare, but a flexible fare and D is a flexible fare in UC along with J, with both offering less restrictions for changes etc.

understand although I found it in :

flying club/spend miles/ upgrade/ search for flights

just seems an odd place to put it. In fact an odd place to have anything UC related.