Visual Studio 2012 doesn't convert vs2010 solution?
I managed to 'convert' the solution file to change the line containing '# Visual Studio 2010' to '# Visual Studio 2012' in the .SLN file.
Manual editing of the solution file is not necessary, or recommended. Simply open the VS2010 solution in VS2012, left-click the solution (at the very top of the Solution Explorer), then use File | Save As to overwrite the original file. This will effectively convert the VS2010 solution file to a VS2012 solution file.