I found this at http://graphicssoft.about.com/od/adobe/f/crossgrade.htm
I hope it helps
Yes, Adobe will allow you to perform a cross-grade, or cross-platform upgrade, of your Adobe product, assuming it meets the upgrade qualifications and provided you send them a signed Letter of Software Destruction. The Letter of Destruction states that you agree to delete all copies of the product you are exchanging and also that you will not "sell, transfer, give away, donate, or otherwise distribute the exchanged Adobe product or copies to anyone else."
You will still need to pay the upgrade cost, and you will need to conduct the purchase via telephone by calling Adobe Sales at 1-800-833-6687 (see Adobe.com for international numbers).