The product code on my s3 was GT-I9300MBDO2U when I unlocked and degraded the phone using z3x when I plugged the phone into lies it said phone not supported. I changed the product code to GT-I9300MBDBTU using the z3x box, I kept the imei and s/n original as on the sticker under the battery. Now when I plug the phone into kies the phone is recognised and will update as normal. I've also done the same to my I9505 which was on orange.
Hope this helps
Sent from my GT-I9505 using xda app-developers app