Depending on if they kept the solution file structure similar between 2008 and 2010, you just need to create an empty solution file in 2008 and look at the top identifier line in it, and copy it into the other solution file.
I however doubt they kept it the same since they were integrating a number of features into solution files.
You would be better off to create a new blank solution in 2008 and use "Add existing project" to reimport them. As long as you dont have any custom "solution" wide references this will work (no solution folders, server setups, etc).
It is probably the fastest and most direct route.
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