Is there a way to override the notification preferences of existent users, not just the defaut notfication preferences for new users??
Theoretically yes, but it might not be a good idea. What if I, as a user of your forum, already have disabled some of the preferences and now you, as the admin of your forum, enable that preference? What good would be a preference setting if my settings are changed by the admin?
You wouldn't be able to do this by sql. You would have to write some function which loops through all users. You would then be able to change the preferences in whatever way you like.