For PSN ID change you'll have to delete your user and create another one to have a new PSN ID. I recommend that you create the new user first so that you can send all the saved data from your old user to your new user so you won't have to start all over.
About changing your username here, sorry don't know anything about that.
13/11/2007
California, U.S | MyPS3, MyWii, MyDS