I found this interesting question in the VSTS MSF Forum that many customers have and answer from Alan Ridlehoover . Heres the answer for it.....
The short answer is no, there is no way to change the process template of an existing project.
Think of it like this: A process template in TFS is similar to a document template in Word.
That said, if you still wish to begin using the CMMI template, my advice is to begin fresh, with a new project. To do this:
Alternatively, it is also possible to modify the individual work item type definitions within an existing project using WITExport.exe and WITImport.exe. Together these programs import and export work item type definitions from an existing project (as XML documents). So, you can export a WIT, modify it, and import it. If you go this direction, you will definitely want to check out the Visual Studio 2005 SDK 3.0 - September 2006 RTM (which you can find here), as it contains the documentation for the XML format you'll be modifying. Note: Changing a WIT for a specific project does NOT update the underlying process template. Also note that going this direction may have negative side-effects, such as broken reports. The safest method is to create a new project as described above.