If you change a username, the name is also changed in the entry-table. The reason for saving it in the entry-table is: if you delete a user, the postings would be nameless. This way, the names remain in the postings.

don't take it personally, but i think this procedure isn't very elegant.
why should an information, i already have, be saved to the db about 100 times more? isn't that data waste?
I'd prefer to add a new column to the user table, in which the user-status (deleted|banned|etc.) could be saved. even if the user-information is finally deleted from the db, what can be said against marking postings with "user deleted"?

Another posibility is: Don't save the username for registered users, just the user ID. When a user is deleted (through the admin panel, obviously, not directly through the DB), add the username to all of his entries and delete the user's data