Sorry, I misunderstood your question. The only suggestion I can make is to go to www.virginmedia/mytivoapps and register again with a new username and put in the pin you want. Maybe someone else can make a better suggestion.
If you go into my apps on Tivo and click on twitter you will find the next screen says My Apps Login with your apps name under "I'm not on this list". click on not on this list and put in a different name and pin and then delete the original my apps name with the yellow button.
You would have to go into www.virginmedia/mytivoapps to activate it.