Microsoft will allow downgrades from Windows 8 Pro to Windows 7 or Vista if customers of its latest operating system are not satisfied with it. This is according the operating system's licensing agreement.
Users can choose to install Windows 7 or Windows Vista in place of the new OS; however, we believe that most users will not opt to downgrade their copies of Windows 8 that are pre-installed with new systems that they have purchased. An exception to this would be mission-critical applications that have not been fully tested on Windows 8 operating system. Systems that are running such applications would have to opt for operating system downgrades.
For users who want to try out the new operating system and do not want to tangle with the hassles of a dual-boot option, they can always choose to use Windows To Go. This option is only available to users of Windows 8 Enterprise. Do note that Windows XP and earlier operating systems are not included as downgrade options. Owners of retail versions and other editions of Windows 8, need not apply. This is because this downgrade service is only offered on OEM copies of Windows 8 Pro. Windows 8 is expected to be launched on 26 October this year.
(Source: PC World)