Hi Ermanno,
You have use event 2681 for your requirement.
This event is used for making changes in the structure C_FKKINVDOC_HEADER in scenarios for actual printing and repeat printing (parameter I_REPRINT), but not for a test print.
This structure C_FKKINVDOC_HEADER contains the Business partner and the contract account no..So as per your business requirement you can make changes to the Business partner and the contract account in this structure.
Hope it helps...
Thanks,
Amlan