Hi,
For changing FA in FI document, please, consult note 1921619 - Reassignment Documents: FKBER not Updated. As you can see, field FKBER is not updated in BSEG, but FKBER_LONG is. This last field is the one which is used in business flows.
For CO documents, I'm puzzled... But, do you have CO integrated in PSM-FM? If not, reassignment tool will have no impact on these documents.
Regards,
Eli