Extracted from the Microsoft TechNet where it mentioned about Windows 7 Upgrade Paths.
The document outlines possible upgrade scenarios from older versions of Windows.
For a Web version of this document, see Windows 7 Upgrade Paths in the Windows Client TechCenter Library.
For IT Professional who is planning your migration, you should start reading up and even try to test out those upgrade path.
Ohh.. The link is
Do not wait. DRU now D- Download, R – Read and U – understand