Both Y and B fares upgrade on KE metal to Z inventory. So if you see Z on the KL flight number then you should be able to call up DL and have them process the upgrade with your GUC.
Basically its like upgrading on KLM... you book with the DL codeshare and call and they'll convert it to the partner flight number and process the upgrade into Z. Just that for KE you need the higher fare Y/B to upgrade to Z.