bbPress.org

add_action( 'init', 'nicenames_to_display_name' );
function nicenames_to_display_name() {
    foreach ( get_users() as $user ) {
		if ( $user->data->user_status == 0 && $user->data->user_nicename != $user->data->display_name ) {
			$user_ids[] = $user->ID;
		}
	}
	foreach( $user_ids as $uid ) {
		$info = get_userdata( $uid );
		$display_name = $info->data->display_name;
		if ($display_name) {
			$args = array(
				'ID'            => $uid,
				'user_nicename' => strtolower(str_replace(" ", "_", $display_name))
			);
			wp_update_user( $args );
		}
	}
}