Tax rates to be changes in scheduling agreement

I have lots of scheduling agreement which have been released as well. Now as per the new budget, we created new condition records and maintained new tax rates (excise duty from 12 to 12.50%).

we have a z program to perform GRN and it has logic written at SA line items basis so whenever we are doing GRN, it is picking old tax rates.

Is there a way i can change tax rates in PO/SA.

any function module is available.