When you upgrade, you don't actually create a new sub-account, therefore it retains the same username, password and contents.
What you are doing is purchasing a sub-account product (not a sub-account itself) and using the product to upgrade the existing sub-account so that it has more storage.
It can be a bit confusing that sub-account products (which are not sub-accounts themselves), and sub-accounts (which are) have similar names.
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