Upgrade from U fare on KLM?

I noticed recently that upgrades using SWU are possible on KLM from a U fare. I am planning a trip to CDG in the Fall and since O availability is pretty much non existent from S or W fare on AF, I was wondering if I would have more chance to go through KL instead connecting into AMS.

Does anyone has any advice, experience to share on that topic? What fare class should I look for in EF to see if upgrades are an option when I book?

Thanks a lot for advising a newly excited PM.