I would presume these groups' paid premium status allows them to create subgroups until they actually exercise their right to revert to
‘Free/Basic Group” status, and that any subgroups they might choose to create with “premium” privileges in the interim you would similarly recognize as “grandfathered"?
Any group that is created before Wednesday at 9am pacific time will continue to have the ability to create subgroups, even in basic form, including all the groups that moved over from Yahoo. No existing group will lose that ability.