Always create a new Patient number if you are changing anything in the entire Patient demographic record. This will insure that your notes and transactions keep all the same information throughout using this number. Think of each Patient number as an individual identifier for that certain period of time involving that Patient demographic information in it's entirety .
UPDATE: If you are just adding a DX code and update the pointer you would not necessarily have to create a new number. Do make sure this additional DX is reflected in your notes. If in doubt, create a new number, or copy the old information to a new number under Specific Activities and put in the new/updated information once the second number is created.
Click here for article on how to copy existing patient details into new patient number.