This is something I've noticed recently which didn't use to be the case:

On the sidebar on the left on the executive club homepage, if you select book with cash and upgrade with avios, you are only able to select Economy as the travel class once you have entered the departure airport. I think this may be a bug because I can still go to "Spending Avios" -> "Book a new flight and upgrade now" to get to the page where you can do the same thing and here all of the classes are presented.

Anyone else notice this? Had me worried initially as I thought you can no longer book with cash and upgrade with avios from WT+!