On a new install of WordPress, I went into the mysql database directly and changed user_login and user_nicename in the wp_users table from “admin” to “carl”. I did this instead of just changing the display_as so that my login would be “carl” and so that the URL for my author page would be “/author/carl” not “/author/admin”.
I had to clear out the cache files in wp-content/cache for the change to take effect properly, but after that WordPress had no problem with the change.
