Is it safe to R&R plugs on a hot engine?

Sure, but you will definitely burn at least one knuckle, and possibly manage a cut as you withdraw the hand in question.

 

If you have a good plug-wrench it will have a rubber ring to hold onto the spark plug, and you can tap it out without touching the hot plug. Be careful when you insert the new plug in the wrench, the wrench will also be getting hot....

 

Let it cool first....probably best to torque them down on a old engine anyway.