Need to update chile record triggered on the update of the Parent

In PO entry we have developed a customization that allows the user to change the plant and warehouse from the PO Header. When this happens, we have a BPM that triggers a write to POREL to update the warehouse on records that exist. This was done on PO.Update using an exclusive-lock.

We recently discovered an issue with this.

In the Base PO Entry screen, when you create a new PO Release for a part that does not exist in the plant the system will build that record for you. Our currently customization bypasses that because it is not using the BO.

I do not want to call PO.Update from within PO.Update because it will throw a "Row has been modified by another user" error. Since the POREL table is not being updated there is no temp table built for it correct?

I have also tried this through C# and it throws an error that the Warehouse is not on record. Doing a trace does not return any info on what happens in the base form.