Switch from Email to Username

How difficult would it be to augment this plugin to rely on usernames instead of email addresses? Where email addresses can be easily changed by users, and are therefore potentially difficult to track (require a manual check to see if the user has changed from gmail to yahoo to whatever), the username cannot be changed after sign-up, so I can always depend on it staying static. I thought I could perhaps just change any call of $user_email to $user_login, but I went in to look, and got lost.