Boost Your BIM has released a free bulk file upgrader add-in for Revit...
"This tool allows you to specify a source folder containing Autodesk® Revit® files and have every file in that folder upgraded to the current version of Revit. All files are saved in a user-specified destination folder."
Currently the add-in supports upgrading files to Revit 2014.
Update 9/1/2014: Upgrading files to Revit 2015 is now supported.
This free add-in is available from the Autodesk Exchange Apps website.