There is no version of Quicken that Intuit lists Windows 8 as a supported OS in the System Requirements.
With that said, users have written in saying that it works and others have had issues, but some of those I believe were issues with unfamiliarity with the new Windows and some permission issues. If at all possible, I would try testing your version on a Windows 8 installation prior to converting/upgrading the OS just to see what happens.
You won't know for sure until you try it. Write back and let us know your findings.