If you have a Parallels Window 7 virtual machine running on your Mac, Parallels suggests waiting for a while to upgrade to Windows 8;
Critical and Urgent Windows 8 Service Advisory
Excited about Windows 8? So are we! Below is critically important information to help you successfully enjoy Windows 8 on your Mac. Please read this entire notice, as it may affect your computer’s performance.
Upgrading from an existing Windows OS to Windows 8?
Upgrading from an existing Windows OS to Windows 8? We strongly suggestwaiting until Parallels has finalized testing the upgrade process. Upgrading now may damage your virtual machine, causing you to lose all your data, files and Windows applications. We’re working hard on completing our testing of the Windows 8 upgrade, and will inform you via in-product notification when you can successfully upgrade to Windows 8. In the meantime, you can check herehttp://kb.parallels.com/en/
Are you planning on performing a full install of any of the Windows 8 versions into a new virtual machine?
This should work just fine. KB114973 has the steps to smoothly and effectively add Windows 8 to your Mac. Be sure that you are using the very latest build of Parallels Desktop 8 to create this new virtual machine. To be sure you are on the latest build, go to the Parallels Desktop menu and click Check for Updates.