Charge a WooCommerce pre-order when the associated order is set to Completed status

sv_wc_charge_order_on_order_complete.php

<?php // only copy this line if needed

/**
 * Charge a WooCommerce pre-order when the associated order is set to Completed status
 *
 * @param int $order_id
 * @param \WC_Order $order
 */
function sv_wc_charge_order_on_order_complete( $order_id, $order ) {

	if ( ! class_exists( 'WC_Pre_Orders_Order' ) ) {
		return;
	}

	if ( WC_Pre_Orders_Order::order_contains_pre_order( $order ) && WC_Pre_Orders_Manager::can_pre_order_be_changed_to( 'completed', $order ) ) {
		WC_Pre_Orders_Manager::complete_pre_order( $order, $message );
	}
}
add_action( 'woocommerce_order_status_completed', 'sv_wc_charge_order_on_order_complete', 10, 2 );