Auto change Woocommerce Subscriptions status to "On-Hold" rather than "Active"

This can be done in two steps:

1) With a custom function hooked in woocommerce_thankyou action hook, when the order has a 'processing' status and contains subscriptions, we update the subscriptions status to 'on-hold':

add_action( 'woocommerce_thankyou', 'custom_thankyou_subscription_action', 50, 1 );function custom_thankyou_subscription_action( $order_id ){    if( ! $order_id ) return;    $order = wc_get_order( $order_id ); // Get an instance of the WC_Order object    // If the order has a 'processing' status and contains a subscription     if( wcs_order_contains_subscription( $order ) && $order->has_status( 'processing' ) ){        // Get an array of WC_Subscription objects        $subscriptions = wcs_get_subscriptions_for_order( $order_id );        foreach( $subscriptions as $subscription_id => $subscription ){            // Change the status of the WC_Subscription object            $subscription->update_status( 'on-hold' );        }    }}

Code goes in function.php file of your active child theme (or theme) or also in any plugin file.

2) With a custom function hooked in woocommerce_order_status_completed action hook, when the order status is changed to "completed", it will auto change the subscription status to "active":

// When Order is "completed" auto-change the status of the WC_Subscription object to 'on-hold'add_action('woocommerce_order_status_completed','updating_order_status_completed_with_subscription');function updating_order_status_completed_with_subscription($order_id) {    $order = wc_get_order($order_id);  // Get an instance of the WC_Order object    if( wcs_order_contains_subscription( $order ) ){        // Get an array of WC_Subscription objects        $subscriptions = wcs_get_subscriptions_for_order( $order_id );        foreach( $subscriptions as $subscription_id => $subscription ){            // Change the status of the WC_Subscription object            $subscription->update_status( 'active' );        }    }}

Code goes in function.php file of your active child theme (or theme) or also in any plugin file.

All code is tested on Woocommerce 3+ and works.