Adding functions to custom workflows

I have modified the delivered Oracle Workflow HR_EMP_TRANSFER_JSP_PRC. Part of the transfer process requires things like GRE and Default Cost Center to be updated when an employee is transferred to a different department or location. I have been able to successfully update the GRE on the assignment record by using a custom procedure that updates the appropriate value (P_SOFT_CODING_KEYFLEX_ID) in the HR_API_TRANSACTION_VALUES table. I have tried that same method to update the default cost center using the P_DEFAULT_CODE_COMB_ID. My code updates the value correctly in the HR_API_TRANSACTION_VALUE table, but unlike the P_SOFT_CODING_KEYFLEX_ID it doesn't update the assignment record. It should