[Bug][SubscriptionBilling]: Customer cannot be changed on contract with closed subscription lines

[miljance]

Describe the issue

When a customer contract contains closed subscription lines (lines marked as Closed = Yes after invoicing past their end date), attempting to change the Sell-to or Bill-to customer on the contract header fails with a blocking error.

This is inconsistent behavior — contracts with terminated lines that are not closed (Closed = No) allow the customer to be changed freely. There is no functional reason why the presence of closed lines should prevent a header-level customer change, and it forces users into an inefficient workaround of manually re-opening each closed line, making the change, and then re-closing every line individually.

Expected behavior

Changing the Sell-to or Bill-to customer on a contract header should succeed regardless of whether closed subscription lines exist on the contract. A terminated line with Closed = Yes and a terminated line with Closed = No should behave identically in this regard — both should permit the customer change.

Steps to reproduce

1. Create a customer contract with at least one subscription/contract line.
2. Set the Subscription Line End Date to a date in the past (e.g. 31.01.2026).
3. Invoice the line fully up to the end date.
4. Run the action "Update Subscription Line Dates".
   • ✅ Result: The contract line is marked as Closed = Yes and moved to the Closed Lines subpage.
5. Attempt to change the Sell-to Customer or Bill-to Customer on the contract header.
   • ❌ Result: The following error is displayed:

     Subscription Lines for closed contract lines may not be edited. Remove the "Finished" indicator in the contract to be able to edit the Subscription Line.

Additional context

No response

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