Change shipping conditions and partners (for CR/SP) in Delivery Document

Hi All,

I have a requirement where I need to change the shipping condition in a delivery document depending upon the total weight of the delivery. Further I need to change the partner (forwarding agent CR/SP) in Delivery document depending on shipping conditions in the delivery. Which is the best possible way to accomplish this task? I am trying to use USEREXIT_SAVE_DOCUMENT_PREPARE in MV50AFZ1. I am not seeing any effect when I directly update the internal table XVBPA and LIKP available in this exit. So I need some help in coding this so that I can update the shipping condition and partner in the partner function CR (forwarding agent). Any help is appreciated.

Thanks,

Giri..