Hello, I'm a new homeowner and I never installed a thermostat before. I bought 2 Nests since my new house have one on the main level and one on the top floor. I watched some installation videos on youtube but I'm a bit iffy since all the video I watched just dealt with installing one Nest. Can someone please answer my questions below?

1. If I shut down the HVAC unit from my breaker but by some chance the unit is still powered will I damage it if I was to remove the thermostat? I'm just not 100% sure that they previous owner labeled everything correctly at the breaker.

2. Anything I should consider? Will have 2 Nests be an issue?

Below are my steps:
1. Shutdown the HVAC from the breaker.
2. Switch the furnace off.
3. Remove the old thermostats and check to see if my current system is compatible with the Nest.
4. Take pictures of the wiring before installing the Nest in case I have to revert back.
5. Install the Nest.