Illegal user names in User Registration option
 

Does adding a new name to the list of illegal user names in User Registration options have any effect on users who have already registered with that as part of their name? For example, I want to prevent user names containing cialis although I have a couple of acceptable users with specialist already in their names and I don't want them affected. I realize that new users can't register with specialist variations, I just don't want the current users affected.