I get asked or nagged about this every week—people want to work on the same projects and solutions in VS 2008 and 2010. Generally speaking, there is only one attribute in one XML tag different between the two files, but there may be other differences in project support (for instance, SharePoint projects). Steve Dunn—who I don’t know from Adam—has put together this command-line tool to switch that attribute.
Steve describes it as a “use at your own risk” kind of tool. I’d have a backup copy of everything in version control, just in case it mangles something.