VBOF is updating the conditions (Overwriting the condition values)

Hi Experts,

Need your help on the below issue with Rebates in ECC 6.0

We have set up the below steps for rebates.

1. Rebate agreement is set up ( Say 2 %)

2. Rebate accrues at the time of Billing document  ( 2%)

3. Settle the rebate for higher amount

4. Now, we have updated the rebate agreements with 3%

5. Execute VBOF ( Update billing documents)

After step 5, system is

1. Deleting the earlier rebate conditions @2%- See step 2 above.

2. Creating a new condition @ 3%- see step 4 above

Though new accounting documents are created, there is no trace available for changes in SD Billing documents.

Can we anyway activate so that system will only updates the delta condition values in the billing documents instead of deleting the earlier condition and recreating a new one.

Thanks in advance for your help.

Thanks

Siva