Set a Discount code while updating a Subscription

[frankie567]

We had a case where a customer subscribed to a product with a one-time Discount code.

Then, the merchant wished to update the Subscription to another pricing in the same month. They expected the Discount code to apply again on the new invoice. Since it's a one-time Discount code, it's not the case.

The behavior is then consistent but it would make sense to allow to set a Discount code while updating the Subscription (even without changing the price).