Downgrading Apple One Subscription and Adding Separate iCloud 2TB plan

This support article implies you can have iCloud+ along with Apple One that may help in this transition.

What happens to your iCloud storage when you sign up for Apple One - Apple Support

"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+ plan, your family can share up to 4TB of total iCloud storage."

I haven't done that but from my reading of it you can sign up for 2TB iCloud and then cancel the Apple One plan, that way you have continuous coverage.

If you still have questions or problems:

Refer to this document for ways to contact Apple --> Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support

What I am saying (because I don't have Apple One, I just read documentation) is it appears you can also subscribe to iCloud+ @ 2TB and have Apple One. That means that when you cancel Apple One you have iCloud+ already in position to cover your iCloud needs without any interrupted coverage. Or that's how I read it.