Feature suggestion.
Allowing users to change their username once.
You could add a a column to the user's table and call it "oldusrname". Then have a page where you can change it and you swap the "username" data into "oldusrname".
You will just check if "oldusrname" is empty to allow them to change it. And if it's not empty, give them the option to revert back to their old username but only now have an option to go back to the new username they entered, not create a new one.

I'm not sure if statuses have a poster_id column or if you just spit out their username in the status table but it would need to use IDs.