Hi Andrei,
True it is a warning message and I tried BAPI_OUTBDELIVERY_CHANGE with 0 quantity and I am getting error saying "Delivery quantity must be entered for the item".
when I am using IDOC to post PGI with the 0 quantity it is processing despite of warning message, but when I use WS_DELIVERY_UPDATE_2 in my program, it is raising message of type I as error.
Thanks,
Kishore.