When you change signal ref on the existing Event node the old one remains in the process definition. There is no way to remove the old signal definition other than directly editing the XML code.
The problem is that I don't have this "Signals" tab. I used to have it in the previous versions of Designer, but since some time it disappeared. When I compare my current plugin.xml to the one from 02.2013 I can see that these sections dissapeared: