What happens to your personal iCloud data if we downgrade our family share package from Premier?

I already have a family group. I already share TV & music and I'm considering upgrading to Apple Premier.

Going in is easy and appealing. But I don't want to move to Premier if it's a mess to leave or downgrade our package.

CRITICAL: What happens to each family member's personal photos, notes, documents & data stored in the shared 2T storage if we downgrade to Family Apple One with less storage or disband the family group all together?

Before downgrading or disbanding should each family group member establish their own personal storage account that fits their needs? Does their data move automatically or require doing something more than buying a new personal storage plan?

I understand what happens to music, TV, and App Store purchases . But personal data in the shared iCloud storage is murky.