Yes, the old barcode cannot be visible and the FNSKU label should be placed over it. If possible, gently try the remove the old barcode but if not cover it with the new label.
The same goes for a UPS label when you are forwarding a product.
Can I leave the the shipping label my supplier used to send me the prodcuts on the box?
Should I take off manufaturer barcode if I'm putting Amazon codes on?