Not my day. The response I got on my second attempt: "we are no longer able to change codeshare flights to true Lufthansa flights"
Yeah I did this about two weeks ago after booking the flights as a UA code share instead of LH. I explained a friend was applying a GPU and she converted all five flights in about 5 minutes (and I'm only a silver). But, I've noticed lately, every time I call United for the same thing I get different answers. I'd just call once or twice a day until you find someone willing and able to handle it...