It seems like if you have an Apple One subscription and share your storage with other people in a Family Sharing group, you cannot add storage (i.e., 2 TB plan + 2, 6, or 12 TB plan) but only replace (pay for Apple One, including storage, but only access 2, 6, or 12 TB paying the full iCloud+ price for each tier). Yes? No? Apple does its best to not make it clear.
