First take a backup from your project;)
Yes. Changing the solution file, as you said in your question, works great! But every time you want to open in a different version of visual studio, you must make this change in the solution file.
This means that if you opened the solution in visual studio 2010, and now you want to open it in visual studio 2008, you must change these two lines, and then visual studio 2008 will open it! also, if you want to open a visual studio 2008 solution in visual studio 2010, you must change these two lines.
Note. For the first time, Visual Studio 2010 can show a transformation wizard. just click the "Finish" button.
And remember, do not change the target version of .Net Framework to 4.0 (or higher!)
I tried it right now and it worked. (I modified the solution file using Notepad ++)
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