Fields accepted when updating a discount. Two lock-out rules apply, in this order: once the discount has been redeemed on an order its money-affecting fields (amount, percent, discount_method, applies_to, apply_from, the scoping arrays, limit_1_per_customer, discounts_per_redemption) are locked and return 422, while renaming and expiring still work; a discount that is already expired freezes its other fields and returns 422 (Discount cannot be changed because it has expired). Expiring is reversible - expired: false clears the end date and a future apply_until pushes it out, either of which makes the discount live again - so editing an expired discount means lifting the expiry first. Expiring a discount and renaming it in the same request is fine; it is a rename sent to a discount that is already expired that gets refused.