Before somebody marks this question as a duplicate of this question Can django\'s auth_user.username be varchar(75)? How could that be done? or other such questions on SO, pleas
Another solution is to change your authentication code. When a new user signs up, they enter an email, you save this in the email field so you have it, and if their email is >30 characters, you shorten it before putting it into the user field. Then, change your login authentication for new logins.