Registry Edit vs. New

Trying to edit Windows XP registry in our ZDM 7 via Application Object using Registry Distribution Option. I'm fuzzy on how to handle an edit/change to an existing DWORD value versus simply adding a new Key entry and DWORD value. I seem to able to add new but not edit existing entries to the workstation associated. What are the gotcha's? I see the "Item will:" drop-down but not certain precisely how to utilize to make the change.