Hi,
When an order is created, it is independent of the changes you can make on your products, discounts, coupons, etc since it's a contract between you and the customer.
If tomorrow you decide to change the price of your products on your store, the prices of the orders in the database shouldn't change... Well, that's the same here. If you want to change an order coupon value, you will have to edit it directly via the "sales" menu and change that there with the small edit icon next to the coupon value.