If you need to change the Disqus shortname being used on your WordPress site, you'll need to reset the Disqus plugin.
login in as admin for your WordPress site.
click on "Comments" in the admin sidebar.
click "Plugin Settings" in the upper right corner.
scroll down to the bottom of the plugin settings page.
click the "Reset" button under "Reset Disqus".
login with your primary moderator credentials.
choose your preferred shortname from the list.
Tip: If you encounter an error that says you must upgrade your database to continue, click "Upgrade" and continue – you will need to click "Reset" again.