Code to retain Sequential number for old orders even after deactivating the plugin - By WebToffee

functions.php

<?php //Do not copy this line of code 
add_filter('woocommerce_order_number', 'wt_show_old_order_number', PHP_INT_MAX, 2);
function wt_show_old_order_number($order_number, $order) 
{	
    if ($order instanceof WC_Order) {
        $meta_value = $order->get_meta('_order_number');

        if ($meta_value) {
            $order_number = $meta_value;
        }
    }
    return $order_number; 
}