Don't upgrade to Windows 8 under Parallels without reading this first. Ignore this advice and you could damage your virtual machine, according to Parallels.
This looked important so we thought we'd mention it.
Parallels lets you run Windows applications on a Mac without dual-booting. They behave just like Mac applications, although they don't look like them - they still look like Windows apps.
It's clever stuff, and heaven knows what goes on under the hood. It's not surprising, then, that when the time comes to upgrade from Windows 7 to Windows 8, on your Mac, things can get a bit tricky.
Parallels is still testing the upgrade process. Meanwhile, they strongly advise Mac users running Windows under Parallels not to upgrade yet, but to wait until Parallels says it's safe to do so.
Here it is in their own words:
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?
We strongly suggest waiting 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 here for status updates.
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.