You cannot convert higher versions to lower versions, as some libraries and assemblies do not exist in lower versions, but you can do the opposite.
In other words, you cannot convert XNA 4.0 to XNA 3.1 (or you cannot open XNA 4.0 in C # 2008), but you can convert XNA 3.1 to XNA 4.0, but still you need to check some changes though.
Hope I helped.
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