Yes it is possible to downgrade from VS 2010 to VS 2008, and it's very easy.
1. Make back up of your .sln file
1.1. If under source control, check out the solution file only (source control explorer not from solution explorer).
2. Open your VS 2010 .sln in notepad.
It should look something like this:
Microsoft Visual Studio Solution File, Format Version 11.00 # Visual Studio 2010 Project("{FAE04EC0-3 ...... ...........................
3. Change the Version 11.0 to Version 10.0 and Visual Studio 2010 to 2008
It should look something like this:
Microsoft Visual Studio Solution File, Format Version 10.00 # Visual Studio 2008 Project("{FAE04EC0-3 ..... ..........................
4. Save the file.