I think MPU talked a few episodes back about getting 4TB of storage on iCloud, but that they couldn’t find how to get that much storage. I found the answer finally.
If the customer is already enrolled in an iCloud+ storage plan that has more storage than they would be receiving in the Apple One plan, then the two storage amounts stack together and they’ll still pay for the separate storage plan.
However, if the customer is already enrolled in an iCloud+ storage plan with less storage than is in their Apple One plan, the separate plan will be canceled when they sign up for Apple One.