You need to import the messages back into Outlook and then save them again with the new naming options.
- Create a temporary folder in Outlook.
- Copy and paste (or drag and drop) the messages you want to rename into the temporary folder.
- If you want to save the messages in the same location then move the original messages somewhere else (you can delete them once you are happy that the messages have been re-saved correctly).
- Open MsgSave, set the naming options and then save the messages to the desired location.