It depends on a kind of project you use. It's absolutely impossible to convert VC++ back from 2010 since they have completely different format and no tools to rollback. However, C# and VB.NET still use the same msbuild project format as in VS2008, which will be changed in the next VS version (Dev11) and so all you need to manage C# or VB.NET project version is the ToolsVersion attribute of the Project element.
VS 2008 C# project version:
<Project DefaultTargets="Build" ToolsVersion="3.5" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
VS 2010 C# project version:
<Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
One of the approaches is to use both project systems simultaneously as our team does. Another one is to use targeting for different .NET frameworks in VS2010 projects. V2010 supports all previous .NET frameworks as targets farmeworks till .NET 2.0
Finally, don't be upset by using VS2008 as it's much faster and reliable.