I was considering switching from PC to a Mac, but I have Adobe CS4 Design Premium and didn't want to pay the $1,799 to buy it again. It was surprisingly difficult to find any information about whether or not I could do it, and if so how. Some Adobe employees at a trade show even told me that it couldn't be done.

It turns out, though, that for $6.25 shipping and handling and the promise that you'll stop using and destroy your older version, they'll let you change. They call it Cross-Platform Swap. Here's how you do it:

1. Go to http://www.adobe.com/go/supportportal If you have an account, log in. If you don't, you'll need to create one.
   
   
2. Under Get Support, Click on Orders and Returns
   
   
3. Choose the issue type Return / Exchange / Refund and then click the button that says Proceed to Online Form
   
   
4. Fill out the form, typing Cross Platform Swap in the subject line. Submit.
   -- In the notes field, here's what I typed: I am switching to a Mac and would like to do a cross-platform swap. I understand that my existing copy must be destroyed as soon as the new one arrives.
   
   
5. The next day, you'll get an email saying your issue is resolved and providing no useful information. Instructions for what to do next are actually in the ticket itself, which you can get back to on the support portal. Here's what you do:
   -- Call 1-800-833-6687 and follow the prompts to customer service (#2 then #4)
   -- Have your case number and credit card ready
   -- Give them your credit card info and verify all your contact information
   

And that's it. They charge your credit card for $6.25 shipping and handling. Your old serial number gets invalidated, you destroy the old copy, and they send you the new one.

As a side note, here's a tip for getting customer service from Adobe: Instead of going to the support portal when you have a question, go to the pages on their website where they have prices and information about purchasing products. Almost immediately, you'll get a helpful person IMing you asking if they can help. These IM helpers are nowhere to be found (ironically) in the support section of their site.

Oh, and their customer support phone numbers aren't that easy to find, so here they are:
Adobe Customer Support: 1-800-585-0774 option #4
or
Adobe Customer Support: 1-800-833-6687 option #2 then option #4.
 
Good luck platform-swappers!