Switching to a new version of Windows is virtually a nightmare if you have to manage more than 100 PC. This is why I value a lot a tool like Microsoft Deployment Toolkit 2010 Beta 2. A tool that can be downloaded here.
MDT 2010 is the next version of Microsoft Deployment Toolkit, a Solution Accelerator for operating system and application deployment. New features such as flexible driver management, optimized user interface workflow, and Windows PowerShell™ can simplify deployment and make your job easier. MDT 2010 will support deployment of Windows 7 and Windows Server 2008 R2 in addition to deployment of Windows Vista®, Windows Server 2008, Windows Server 2003, and Windows XP