What category of components can you replace in a computer without turning off the power?

go to the little circle thing in the bottom left side of your computer, click it. One of the options is restart.

Plug-and-Play.

If you restart it in the menu with the mouse or keyboard then it's a "Soft" or normal reboot. If you have to restart with the power button (or restart button on older Macs) on the side of computer...

Reset

Only if you're very unlucky and the switch bounces; turning on the power is more likely to cause damage as components are cold and suddenly receive power, heating them up.