I'm curious... If I were to set up a coupon that zeros out a membership price when people sign up, will they automatically switch to the plan price and begin billing once that month is over? Also, can coupons be applied to existing accounts like "credits"? The payment processor is Paypal.
No, it is impossible. With PayPal recurring, coupon discount will be applied to subscription price (for future charges too) No, credits are not supported.