Using coupons with subscriptions – what happens at renewal?

Our memberships are all annual with automatic renewal.

1. If we do a coupon to give “credit” to folks who are on our old system when they join us on the new system, will the Paypal subscription be for the full price or the discounted price?

2. It appears if we do a 100% discount to start, then no Paypal subscription is set at all since the Paypal button disappears and the user is not logged in, etc… So in this case, what happens at renewal time?