If you're using Camtasia 3 on Mac you should already have a Key for Windows 9. If not, Tech Support can help you get a Key. If you're using Camtasia 2, when you upgrade, you get a Key for both Mac and Windows.
But forgive me if I take issue here. Having used both Mac and Windows, I don't consider a switch to Window to be an upgrade. However Cam 9 for Windows does have more features than Cam 3 for Mac