For an existing composite test, changing the test type to something else, e.g. simple numerical, will throw an error, even if the code snippet is empty.
This is related to issue #168: if a test has any history then its test type can't be changed (unless you're seeing another bug that I'm not aware of). I still think I prefer to fix the test type for anything with existing test instances as a small measure of preventing rewriting history. Certainly we don't want tests changing from a string type to a numerical type, but if you have a good case for changing between numerical types for tests with existing data I'm willing to be convinced.