There shouldn't be any problem while upgrading and switching the OS.
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.
Is Adobe still making people jump through these hoops? I have to confess that I find this completely ridiculous. In terms of licensing, it shouldn't matter to Adobe whether or not you install the software on a Mac or on Windows. Lightroom installs on Mac and Windows with the same license key. There is absolutely no good reason why this shouldn't be true for CS6.
I'm currently working on Mac but will probably switch to Windows PC in the near future (at least for the Adobe suite). My experience with Premiere Pro CS5.5 on the Mac hasn't been what I expected. Hopefully
CS6 is more stable and responsive. With Avid Media Composer (which I also own) you don't have to worry which OS version to buy. They provide you with both, Mac OS and Windows licenses in the same
package. Of course you can only use one at a time, but if you have a Mac and a Window PC computers at the shop you can have them both installed on each computer and then simply activate and deactivate
the software whenever you want it. I know that now with the Adobe Creative Cloud, switching platforms is easy, but the same flexibility should be included in the boxed version. That is what the competition
(Avid) has been doing for a long time.