Hello, Mitchell Gross! Thanks for reaching out to Apple Support Communities.
We've reviewed your question and it looks like you’re looking for clarification on how to save all data in your family’s iCloud accounts, after moving to an Apple One account. It's understandable that you'd want this to be arranged without data loss, so allow us to assist with that. Please start by taking a look here: What happens to your iCloud storage plan when you sign up for Apple One. When you share an iCloud storage plan, family members can't see each others' photos, files, or documents. They will not lose their content.
There is no need to cancel their accounts to enjoy the benefits of Apple One iCloud storage sharing. If this does not resolve the issue or if you have follow-up questions, please let us know. We're here for you. Have a fantastic day!