I agree with Roxor that you cannot change a request to a different type after it has been submitted. Your method of making the request type editable can give the appearance of changing the request type but this will not change the workflow or the mapping of any of the fields on the request. Unless you are very careful to make all of your request types use exactly the same fields and have them stored in exactly the same parameter fields, you are going to see some very strange behavior.
You could use the ksc_copy_request special command to create a duplicate request with fields mapped according to token names, then close the original request. This could not be done from another request, though.
Actually, PPM OOTB functionality DOES allow you to change the request type of a request, but what is actually does is creating a new request of the <new request type> and copying the corresponding fields.
--remember to kudos people who helped solve your problem