Interesting since C+ books into W or S... so when it says originally purchased fare class, does that mean W or S? Or the underlying basis that the fare is constructed upon?

Trying to explain and have the vast majority of travelers understand that just isn't going to work. If I purchase C+, then I get a ticket that appears in as W or S fare in my itinerary and in the confirmation email. Its not reasonable to expect pax to dig into the nuances of the email and decode the fare basis.

I wonder if this case is since it sounds like the OP bought-up to C+ after buying the main cabin ticket and the agent isn't looking at that but the original non-upgraded fare. I thought if you buy-up then you're supposed to get the benefits of that cabin/seat including SDC.