Can't map to order_id to update existing orders

I needed to update existing orders, which were not imported using Migrate, using $this->systemOfRecord = Migration::DESTINATION; and mapping $this->addFieldMapping('order_id', 'order_id')->description('Drupal Order ID.');

Copying the code from ✨ Can't map to product_id and update existing products Closed: outdated I've made this work. Patch to follow.