In this situation, are you obliged to change high-beam lights to low-beam lights?

Yes, when approaching a vehicle travelling in front of you, you are obliged to switch from main beam to dipped headlights. Your main beam headlights will reflect in the preceding car's mirrors and dazzle its driver, creating a hazard. This obligation arises from the provisions of Art. 51 para. 3 of the Road Traffic Act.