Catching the new order status by programming

[Erwans75]

Hello
I'm setting a website with Joomla 2.5.7, VM2.0.12 and Post Affiliates Pro 4.
I want to track the Order status change from manual back-end.
But I do not find where(in the administrator\components\com_virtuemart, I suppose)
Can anybody tell me how I could catch a (manual) change of an order status and catch the old and new one. A php file or any other file.
Which function ?.
Then I could be able to update PAP 4.
Regards
Erwans