What is hot swapping? Hot swapping is when you engage or disengage your lens or memory card without first powering down your camera. The question is must you power down first and whether there are consequences if you don't.
We spoke with Canon, Sony and Nikon and they all said yes and highly recommend that you power down first when changing lenses. We consulted with an engineer and he concurred. So what's the reasoning? The camera body provides power to the lens for things like focus and stabilization. In the process of mounting or unmounting your lens, you may create an electrical arc which can potentially damage your lens, camera or both. He said with a bayonet mount, which is what all cameras use, it's a particular problem. He admits that although the probability is very low, it is a measured probability; therefore the manufacturers must recommend that you power down and he recommends it too.
So what about the memory card? On most cameras when you open the door for the memory card, it powers the camera down automatically. However, if your camera is in the middle of a write and you shut off the power by opening the door, then you will lose that image. When you power down the camera with the on/off switch, it will continue to write until the process is complete unless you open the memory door and interrupt the process. There is also the potential of corrupting the card if you cut the power while the camera is still trying to write to it. Please note that all the manufacturers spoken to recommended that you always power down first.
Shocking Fun Fact: What about static electricity? You may have read that the main reason for turning off your camera before changing lenses is to prevent static electricity. Static electricity is blamed for attracting dust into your camera and onto your sensor. To answer this question we consulted a PhD physicist from the University of Michigan who informed us that although the batteries in the camera are driving electrical currents, which could shock you if you ground the current by interrupting the flow, the electricity itself does not cause static electricity.