flin is correct. The DD types are really a one-way street. There are 3 ways to define them; statically in the config (safest), dropping a model into the Data Dictionary space and running an aspect, and in the code. But once a type has been used, you can't remove the definition until you have cleaned up all instances and related structures. Generally, the static method is safest. The others have not really be tested to death, either.