First ask functional analyst to perform a demo with VA02, if he is not able to change the value, not much hope with the BAPI. (I'm nearly convinced this field is actually calculated from Customizing and changed during life of SO, e.g. already invoicde, material shipped, etc.) -> so note the data changed by functional and try to replicate in BAPI call, of course if requester is unable to change the value...
NB: I'm not sure (not SD/CRM specialist) but the flag you try to change in the BAPI call is CRM related ?
Regards,
Raymond