/Upgrade (devenv.exe)
Visual Studio 2012
Updates the solution file and all of its project files, or the project file specified, to the current Visual Studio formats for these files.
devenv SolutionFile | ProjectFile /upgrade
Backups are automatically created and copied to a directory named Backup that is created in the current directory.
Source-controlled solutions or projects must be checked out before they can be upgraded.
Using the /upgrade switch does not start Visual Studio. Results of the upgrade can be seen in the Upgrade Report for the development language of the solution or project. No error or usage info is returned. For more information on upgrading projects in Visual Studio, see How to: Troubleshoot Unsuccessful Visual Studio Project Upgrades.