Coupon codes not available for renewal payments???!

I’ve today found out that Coupon Codes are not an option for renewals – i.e. the field is not displayed during the renewal process as it is during the purchase process. I use coupons as a way of getting people to renew their subscription before expiration (I don’t use recurring payments) which is an effective way of increasing customer retention.

Does anyone know how to get coupons working for renewals too? If is possible?

Thanks.