wc_hook_horizon_packages.php

// hooking woocommerce purchase to updating the users sub level

function woocommerce_subscription_update_hook($order_id = 0){

    $user_id = get_current_user_id();

    //get product slug from order_id
    $order = new WC_Order( $order_id );
    $items = $order->get_items();

    $product_slug = '';

    foreach ( $items as $item ){
      $product_slug = $item['name'];
    }    

    // need $wc_package_title from wc slug
    $wc_package_title = get_package_title_from_product_slug($product_slug);
    
    $packages = Horizon_Packages_Logic::get_packages_choices();
    
    foreach ( $packages as $package_key => $package_value ){
      if ($wc_package_title == $package_value){
        _log('Setting ' . $user_id . '\'s package to: ' . $wc_package_title);
        //$user_id, $package_id required
        do_action('horizon_update_user_subscription', $user_id, $package_key );
      }
    }
    
}

add_action('woocommerce_payment_complete', 'woocommerce_subscription_update_hook');