Change Bill Plan of a Contract ( BAPI / Function Module ) - Bill Type Field FKARV

Hi
We have a requirement to change BILL PLAN of a contract( Lease i.e. VA42 ) document.( Contract is already created).
On the termimation of servies, we stop the contract( USING BDC) this generates an additonal BIll plan line item( for credit to Customer) if the customer has already paid ahead.
I want to change the Bill Type of bill plan line item(CREDIT Bil Plan Line Item) in the bill plan.( FPLT-FKARV i.e. Proposed billing type for an order-related billing document).

I have looked for
BAPI_CUSTOMERCONTRACT_CHANGE
BAPI_CUSTOMERCONTRACT_CHAANGE

But the interface of these BAPI's does not have anything thru which i can pass changes to bill plan ( FPLT Tables).

I have even tried to modify the bill plan using

SD_SALES_DOCUMENT_READ
SD_SALES_ITEM_LIST_READ

SD_SALES_BILLINGPLAN_READ

SD_SALES_BILLINGPLAN_CHANGE

SD_SALES_DOCUMENT_SAVE

COMMIT WORK AND WAIT.

But the above set does not change anything at the bill plan line item.Though it can change the bill plan at Header level(FPLA).

I know I can change the Bill Type using FM BILLING_SCHEDULE_SAVE, but this is a direct update to DB tables that we dont want to do.

Any help / pointer on this is highly appreciated.


Thanks

Manish