Membership Pro Upgrade Coupon Problem

Hello, I have two membership plans setup and would like to be able to offer a discount when one upgrades from one plan to the other. The problem is the only options available on the account page is to subscribe to the new plan, which takes them directly to paypal. I would like them to be able to enter a code and then go to paypal. Is this even possible, I've looked around and haven't found a definite answer to my problem. I look forward to hearing from you and if you need anything else from me please let me know.