how to set billing block field in VA03 to default using badi?

Hi experts,

I am new to SD area, My SD consultant asked me to create a report.

In that he needs the field "billing block"  to be set to value "space"(i.e the value should be null) in vA03 tcode. Such that once it takes the default value space, it should go and update the same value in table "fplt" for the field faksp. plz see attached screenshot. I tried a sample coding but still its not getting updated.

Should I use   'BAPI_SALESORDER_CHANGE' to change a Sale order then use 'BILLING_SCHEDULE_SAVE' to update the

Billing plan of the changed Sale order.

Tell me if i need to use any user-exits?

this is urgent, plz help me out in this....