I suggest creating a custom item field to store the old UPC so you can still reference it when needed. That way, you can update UPC field with the new UPC without losing the old code. If I recall correctly, you can even make a custom field searchable in Global search. This would allow you to still scan the old UPC to find the correct product.

If that doesn’t work, I would recommend using one of the other native fields in NetSuite like Vendor Name to store old UPCs in. I’m pretty sure that field is marked for searches.

We created an alternate code custom field and made it available for search. The trick however is to make the OLD code the alternate code as it is most likely to become obsolete before the NEW code would be. In this way, the new code will take the primary position of UPC Code, but the old code will still perform a lookup from Alternate Code