I assume you are referring to the Buyer on an Item Branch record. You could try using the Item Branch Update program, R41805, which allows you to do a Mass Update on the Item Branch Buyer field. If you simply want to replace a certain Buyer with another Buyer, this Batch should work for you.
However, if you want to replace the Buyer number on only certain Item Branch records with certain a Supplier, this Batch doesn't have in its view, V4102R, the Vendor (Supplier) number field for you to data select on. Perhaps you could use some other data selection criteria to isolate your records.
Outside of the functionality within JDE, you could always create an SQL update query your change.
