Well, my original WT+ ticket was fully flexible W class. However, on check in for outbound flight they upgraded me to CW prior to check in for free and I noticed that I was then in U class. So I guess that could explain it.

Normally when I get an upgrade due to oversales it's always at the airport, not the day before so this was something of a first for me.

Seems a bit cheeky that they give me a free upgrade and in the process change the terms of the ticket.