Originally Posted by
flatlander
D class can run a bit thin on some flights a few days ahead (eg the Monday 08:30 flight next week is already closed in D class, some others have only D1) so you might need to choose flights that run out less if you are going to buy a WT+ ticket far ahead of time and then upgrade to CW closer to time.
This won't apply if you're just trying to create CW redemption space and do an entirely-Avios redemption (not upgrade-with-avios into CW redemption availability), you can just choose one of the flights with at least D2.
This is important - the joker can't force space for you on every flight - if there is only the most expensive fare class available for the cabin you want (F,J,W,Y) then the joker won't work and they won't usually find space for you. I thought that C was OK in club, but I suspect
flatlander is correct in thinking you need D..