Get the Subscription id from the Order ID
You can use dedicated function wcs_get_subscriptions_for_order() which will retrieve $subscription IDs.
So this could be your code:
add_action('woocommerce_order_status_changed', 'action_order_status_changed');function action_order_status_changed( $order_id ){ $subscriptions_ids = wcs_get_subscriptions_for_order( $order_id, array( 'order_type' => 'any' ) ); // We get all related subscriptions for this order foreach( $subscriptions_ids as $subscription_id => $subscription_obj ) if($subscription_obj->order->id == $order_id) break; // Stop the loop // The subscription ID: $subscription_id // The An instance of the Subscription object: $subscription_obj // ...}