With Windows Vista nearing its first release candidate in preparation for a final launch early next year, Microsoft is providing more details about possible upgrade paths. Of note: Windows 2000 cannot be upgraded to Vista.
While Windows 2000 Professional customers may purchase a cheaper "upgrade" copy of Microsoft next-generation operating system, Windows Vista must be "clean installed," which means users will need to back up their files and data manually and then copy everything into place. Applications will also need to be re-installed.
The x64 version of Windows XP will also have the same requirement. Although the final release of Vista will include both 32-bit and 64-bit iterations of the OS, Microsoft has not designed an upgrade path for the current x64 release.
Windows XP Professional, meanwhile, can be upgraded only to the Business and Ultimate editions of Windows Vista. This is due tof eatures built into XP Pro not being added to the Home Basic and Home Premium SKUs of Vista.
News source: BetaNews