How do I migrate my wife's iCloud from her paid storage to our family storage?
@RDG : That link does not explain how to merge an already existing plan into a family plan that the user has been added to. It merely states that you can:


  If a family member is already paying for their own iCloud storage
  plan, they can choose to switch to your plan or keep their own and
  still remain part of the family. When they switch to the shared family
  plan, they get a refund for the remainder of their personal plan. They
  can't keep their current plan and use the shared family plan at the
  same time.


@Dave Sag: I had the same problem with my family—my son already had a 50G storage plan and after I got a 200G family storage plan and added all my family members to it, for some reason it still showed him as "not using", and nowhere in the iCloud preferences on him MacBook would it allow him to switch to the family plan and get rid of his individual plan.

After a lot of digging I found one article that mentioned that in order to use the family shared iCloud storage you have to be running Mac OS High Sierra (10.13) or higher. Turned out, he was still on Sierra (10.12). After upgrading his MacBook to High Sierra the iCloud preferences now showed a button to "Use Family Storage". Clicking that instantly moved all his iCloud data from his individual storage plan to the family shared storage.

After doing that, it no longer even shows his 50G plan in the iCloud preferences. I'm assuming that it has cancelled that plan. If I find out that it did not automatically cancel that plan and that I have to go cancel it myself, I'll follow up on here to let you know.

Cheers
—Dan