Currently if an exciting clients tries to signup using a promo code intended for new clients WHMCS will show that the discount has been added however when they sign into their account and complete checkout WHCMS will charge them full price without warning.
If we could get add some validation at the checkout, so if a client uses the promo code then chooses to log into an account rather than the order going through and billing them at full cost to show message saying something "The promo code you've entered is for new clients only. Would you like to continue at [normal cost without promo] or contact us for assistance." Gives them the option to continue at full cost or contact us where we maybe able to offer a one-time special discount.
Should save time on refunds and chargebacks.