Manually changing an account in the Item Record would generate this pop up message: "You have changed the account(s) for this item. Click OK to update the accounts on existing transactions, or Cancel to stay on this page."
An update of an account in the Item Record through CSV Import, would update all existing transactions posted. Even transactions created on a closed period will be updated thus resulting to a prompt. You will not be allowed to change item account via CSV if the item has transactions posted on a closed period.