If you want to buy CS6 you will need a "Crossgrade". Search on that term.
I believe CC will work on either platform.
I'm pretty sure nothing special will be required to upgrade from a PC license for CS5 to a Mac license for CS6.
If you sign up for the Creative Cloud, you'll be allowed to run Photoshop CC on both platforms.
Hello! You might have to purchase an upgrade for the same platform, then ask for a swap. See http://helpx.adobe.com/x-productkb/policy-pricing/order-product-platform-language-swap.htm l?PID=2159997 but before doing anything, contact customer service for more information.
Thanks to all who have replied. I appreciate your help.