Just a thought...
If a list is falling back to Free from Premium... and might not like it, they may take 30 days to really discuss it, and 30 to act.
So maybe Archive their SubGroups etc, while allowing to see but no access, or perhaps as a teaser, no access but Read Only.
If a group loses subgroups because of the downgrade, the subgroups are locked, not deleted. You can still see the archives, but it's all read-only.