When you create a listing, Amazon’s system locks the Product ID (UPC, EAN, or GTIN) into the ASIN’s identity. This identifier becomes part of Amazon’s internal product mapping structure.
That’s why you can’t simply “edit” the UPC on an existing ASIN, changing it would technically mean changing the product identity itself.
The only way to correct it is to create a new listing with the GS1 barcode and then merge the two ASINs under catalog support supervision.
