How to downgrade Visual Studio project version

Here is the method to downgrade your VS project version.

1. Make back up of your .sln file
2. Open your VS .sln in text editor.

It look something like this:
Microsoft Visual Studio Solution File, Format Version 11.00

Visual Studio 2010 Project("{FAE04EC0-301F-11D3.......}

1. Change the Version ?? to Version ?? and the Visual Studio yyyy to yyyy

without the version ,you can reference to a project (Or using VS to create a new project),take a view with text editor to get it's.