Swapping High voltage breakers; change 50amp to 40amp
The metal bar that the breaker connects to is hot.  The breaker provides power to the connected wires, and cuts power to them when it is off.  So, if everything else is working correctly, the wires connected to the breaker do not have any power.

While it's possible to not have a main breaker in that panel to cut off all power, there's normally a switch or a breaker in another box that will do it.  The bus bars inside a panel are large and easy to touch, so its best to turn off power to the whole panel when swapping breakers.  Cutting off all power also keeps you safe from wiring mistakes that could result in power being fed back into the panel from another, seemingly unrelated breaker.