bbp_update_reply_position( type $reply_id, type $reply_position )

function bbp_update_reply_position( $reply_id = 0, $reply_position = 0 ) {

	// Bail if reply_id is empty
	$reply_id = bbp_get_reply_id( $reply_id );
	if ( empty( $reply_id ) )
		return false;

	// If no position was passed, get it from the db and update the menu_order
	if ( empty( $reply_position ) ) {
		$reply_position = bbp_get_reply_position_raw( $reply_id, bbp_get_reply_topic_id( $reply_id ) );
	}

	// Update the replies' 'menp_order' with the reply position
	wp_update_post( array(
		'ID'         => $reply_id,
		'menu_order' => $reply_position
	) );

	return (int) $reply_position;
}