I need a bit of clarification on the coupon feature

In Pro Sites I want to offer a coupon code to use at check-out for $30. I am currently set up for subscription payments via paypal. So if they go to sign up for a $39/month package then it's clear they will only have to pay $9 for the first month and then $39 the remaining months. What happens when a person uses that for a $9/month sign up. How does that work? Secondly what if someone is already a client and has been paying monthly and I want to offer them a coupon without interrupting their normal paypal subscription? thanks