Hi,
The best place is a plugin with the trigger "onAfterOrderUpdate".
By creating a plugin you could be able to update HikaShop without loosing your modifications (and so, your feature).
www.hikashop.com/en/support/documentatio...l#onAfterOrderUpdate
The $order object contains "$order->old", so you can check for order status modification thanks to "$order->order_status" and "$order->old->order_status".
If "$order->order_status" is empty or not set it means that the order has been modified by the status is not modified.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.