Please, check if the following notes are implemented:
1966743 - Old account assignment field values not cleared when account category is changed
2090748 - BADI BBP_DETERMINE_ACCT not called on change of account assignment category
Additionally, in your FMDERIVE rules, make sure that you have an 'overwrite' flag on the relevant rule switched on. Otherwise, if your commitment item is already filled with the previous value, it wouldn't be overwritten, when you change account assignment.
You can use the BBP_ACCOUNT_CHECK_BADI to clear the fields passed to ECC in order to redetermine the commitment item. For more information check the following Wiki: