This is a nice little plugin that can help admins change a user's username if needed. Since it can also send an email to the user, that saves some work as well, making updates fairly quick.
One of our clients was generating usernames automatically from First Name and Last Name fields but married couples put in long strings and that made for unwieldy usernames.
Out of the box the "Send the user an email about their updated username." feature does not work properly. Specifically, the plugin's "include/mail.php" file (esp. the "From" address) needs to be customized for your site. We ended up customizing the entire file for our use (including adding a "BCC" header so we get a copy of the email sent). If you do customize this file, you would be well-advised to store a copy of the file somewhere in case the original gets overwritten with a plugin update (or reinstall).
Once mail.php is customized to your satisfaction, this is a great little tool.