Can a Plugin Override New User Default Role Type

You can use the user_registration action to set a custom role directly after wp_insert_user() has been called.

add_action('user_register', 'foo_set_new_user_role', 9999, 1);
function foo_set_new_user_role($user_id){
    $user = new WP_User( $user_id );
    $user->set_role('your_new_role');
}

You can also use the action profile_update for just that, profile updates. It takes two parameters $user_id and $old_user_data.

Hope this helps you out.