One of the world's possibly most expert programmers in C# and .NET, Jon Skeet, the author of "C# in Depth," (imho a "masterpiece" of technical writing) discusses a similar situation here, where a user wants to open a VS 2010 project in VS 2008:[
^].
Visual Studio has been, so far, backwards compatible between versions: you can open a VS 2005 solution in VS 2008, for example, and there have been automatic converters that start-up when opening a solution created in an older VS in a newer one that take care of adjusting project structure, report incompatibility errors, etc.
However, if you start a new solution/project in VS 2005 of the same type (WinForm, Wpf), and import the project files from the VS 2008 solution, under the following conditions you may get the VS 2008 solution to open and build with some fiddling:
1. don't move over the 'bin or 'object folders from the VS 2008. (obvious, huh)
2. the solution in VS 2008 should have been set to compile against the FrameWork version you can compile against in VS 2005.
3.the code in the VS 2008 solution must make no use of any newer features of the FrameWork than those supported by VS 2005.
However, the fact you could do this, doesn't mean you should do this !