what is the reason why we can not open a project created in VS2008 using VS2005?

Why Microsoft do开发者_运维问答es not support Forward comparability?

What you describe is actually forward compatibility.

The reason is that VS 2008 may (hypothetically) include settings in the solution file which change the rules of the game for how the solution should be processed, using mechanics that were not invented at the time of VS 2005. Assuming that these settings can break the solution if not processed correctly, what should VS 2005 do when it encounters them?

That's not backwards compatibility it's forward compatibility. You can open a VS2005 project with VS2008 (backwards compatibility).

Have you tried simply changing the version number at the top of the file? (it's one of the first 3-4 lines IIRC)