Traditionally, if you were to test a preview release of a new version of Windows, you would need to do a clean install of the OS when the final build was released. Not anymore. For Windows 8, Microsoft is allowing those who used the preview release to upgrade to the final version for $39.99.
According to a FAQ released by Microsoft, personal data files will be carried over during the upgrade. Applications, user settings, and user accounts, however, will not.