The project file uses an old format.
When you change a project, close a project, or check a project into source code control, Visual Studio inspects the project format. This error message appears whenever the project file is still using an old format.
In order to use source code control or save changes to your project, you must upgrade to the current Visual Studio project format.
To correct this error
Click Yes to upgrade to the new project format. Clicking Yes will continue with checking in or saving the project.
Click No to avoid upgrading to the new project format. Clicking No will cancel checking in or saving the project.