Going to the module's Public API folder, there, you will see the Arguments folder. Find the typedef for the arguments that you want to change and modify it. Now, you need to make sure you modify the Request, and where the Event handles the request. Same for the reply. A way to do this is to right-click on the typedef on the project and select to find callers, go to each one of the callers and make sure the new elements are handled correctly.
After the modification, you can run the module validator to check everything is ok (DQMH 6.0 now checks if the bundle in the EHL is not missing a wire).
1)I don't think it's possible in DQMH 5.0 to modify arguments of one Event without removing it and re-create it. Lots scripting but very useful. Perhaps, in the convert tool menu?
2) Remove automatically all vi's and controls create during event creation if there is a problem during this process. I have encountered this problem and I have lost time before understanding why I couldn't recreate the same event.