Hi Marionneedshelpplease,
There shouldn't be any problem while upgrading and switching the OS.Please go through this link : http://helpx.adobe.com/x-productkb/policy-pricing/order-product-platform-language-swap.html
If you want to upgrade from CS5 Win to CS5.5 or CS6 MAC, you can do that. Its called Cross Platform Upgrade.
But as Bob mentioned, you need to contact adobe for that. It cannot be done online from adobe website.
For the best assistance, I recommend our chat support at http://adobe.ly/yxj0t6. Our chat representatives can provide a personalized experience to resolve the issue you have described.
Regards,
Rajshree