Apple One family plan
How to get family members off of their own storage plan, and unto our new family storage plan
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Best reply
They will need to make the choice.
See:
What happens to your iCloud storage plan when you sign up for Apple One - Apple Support
If a family member subscribes to Apple One
If you belong to a Family Sharing group and a family member subscribes to an Apple One Family or Premier plan, you can choose to keep your current iCloud storage plan or use the shared iCloud storage in Apple One.
If your family needs additional iCloud storage, the family member who purchased Apple One can buy more iCloud storage. With both Apple One and an iCloud storage plan, your family can share up to 4TB of total iCloud storage.