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