Hi,
you can do this by a hook. I choose the hook AcceptOrder.
In this hook you get the orderid and can query the order via the internal api with the getorders-function.
Within this Array you get the serviceid in ARRAY['orders']['order'][0]['lineitems']['lineitem'][0]['relid'].
With this serviceid you can finally update the product of the customer with the internal api-function: updateclientproduct
This is the way I found out, but I am not really sure if there isn't an easier way to do so. Maybe annother hook. I generate API-Keys via this. This is why i choosed the hook AcceptOrder.
Regards,
Matthias