Why do my old lunch friends now treat me like management?

You took the new role. Maybe it was offered. Maybe you wanted it. Now the people you used to sit with at lunch watch what they say around you. They are not wrong to do this. You hold something new now — notes on their teaching, maybe a say in their evaluations. That is a real change. It is not in your head. You may want them to see you as the same person. That is not unreasonable. It is impossible. A person with power over others' work cannot be seen the same way. No amount of kindness at lunch changes that. Wanting it otherwise wastes your attention on something you cannot control. What you can control is how you use the role. Coach with honesty. Protect their trust, even without their old friendship. Let go of needing their warmth to feel good about your choice.