What for an "Add New Detail" button on the order settings screen? Why there is an error for some of the orders:
Detail Line cannot be added to Order in Status 'XY'.
The button "Add New Detail" does not add any provisioning items to the order, it modifies billing information. The way to use a button is described in the documentation.
The presence/absence of the button depends on the order status and the status configuration in the order flow. When the provisioning is completed, it shall not be possible to add a new detail to the order. An order status property
Is Addable is responsible for the possibility. See the kb #119504 for some more details.
Adding a line of type "Discount" will only add a discount to a current order, it will not be applied to future orders related to the subscription.