Apologies for the confusion. I had created two new registered user groups (Free, Paid), but I think I could have just renamed the "Registered" group to the "Free" one as the default. This way if a paid member expires or unsubscribes they get downgraded to the free plan.
My original question was asking what the default behaviour was for CBSubs when it "kicks" a user out of a Paid Group set up in a plan.