Using United miles

Hello,My husband and I are ready to submit our dossier to Ukraine, hopefully within the next week or so. I'm wondering if any of you have used United miles in conjunction with Lufthansa to fly to Kiev. They are telling me that they do not allow any changes once travel begins; therefore we would have to buy a new one way if our return changes, which is most likely will. Has anyone changed their return successfully. We don't care about paying a change fee, just want to be able to change it. Thanks in advance,MW