This API (https://auth0.com/docs/api/management/v2#!/Users/patch_users_by_id) does not allow setting a username on an existing user profile that did not initially have username set. (The API returns a 400 with error message “Cannot update username for users without an username already set” if you try to.)
We initially had users sign up with just email/password. Having now toggled “Requires username”, and given this API restriction, how else should we backfill the username?
Related GH issue: https://github.com/auth0/wp-auth0/issues/343