Screenshot is from an empty new site.
For an existing user the Username field is disabled.
Currently investigating on how we can make it editable, worst case "copying" the user object and roles to a new user with the new username.
Would be good to know why it's disabled?
Not sure why it's disabled.
But the username is used in IChangeTrackable.CreatedBy, IChangeTrackable.ChangedBy and IChangeTrackable.DeletedBy. So maybe it is to avoid messing up the history of content.