Hi Budi,
When using a BADI you are not making a modification to SAP standard, but an enhancement inside and enhancements spot.
I'm not entirely sure about the BAPI in 1511 but alternatively you could also use BAPI: FAR_CUS_OVERDUE. 1511 will have quite a lot of changes so best to check explore the available BAPI's in SE37 or BAPI and test if these still work correctly with on of your customer accounts.
Regards, Andre