I doubt you'll find an official guide or tool, you'll have to do it manually. Personally, I would do the following:
- Start a new MVC 3 Project (MVC3 is available via nuget)
- Copy your files from one to the other
- Fix any compilation issues
- Use the publish site feature to publish somewhere locally (this will hopefully find compilation issues in your Razor views which don't get found during the project build)
- Test the site fully!
Sorry it's not more helpful, but I don't think people generally design for the downgrade process.