Thanks for the reply guys.

It is in fact a WT -> WT+ cash upgrade so they are quoting me the $275 fee + difference in fare.

The full story is because I'm wondering whether I could move to a neighboring city so that I can fully benefit from upgrading to WT+ as there is a club redemption flight available.

So in total I'd like to move to another flight and then upgrade to WT+ in cash and then upgrade to Club using avios. Pretty complicated but love flying club on the overnight flights where I can, especially if I'm paying for WT+!