When user changed his username the page will reload and redirect to login page. At that page field "Email" is autofilled with new username. But there is no reason for that, because there is impossible to log in with username, user should enter his email address.
Steps to reproduce: 1. Login into Bitbucket account. 2. Go to profile page - https://bitbucket.org/@username, where is "@username" is your username. 3. Go to "Settings". 4. Click "Change" link near you username. 5. Change your username and click "Rename" button. You will see opened login page with entered your new username in "Email" field. 6. Enter your password to "Password" field and click "Log in" button. 7. You will get an error message "You can no longer log in with a username. Use your Atlassian account email address instead.".
Actual result After changing username the login page will opened and "Email" field autofilled with new username.
Expected result After changing username the login page will opened and "Email" field autofilled with user email address or it will be empty.