You don't have to turn off the main breaker if you really know what you're doing. If you are not 100% confident, as you aren't in asking this question, then turn off the main breaker. The reason to leave the main breaker on is obviously for your own safety, but also to not bother the homeowner by disrupting all electronics, such as timers, clocks, and computers. The homeowner is very thankful when you don't turn off everything. That being said, your safety is still #1.

junkmailhold-
I'm with bigbird on this. Yes it's possible to do it safely, but, if the main breaker is not off, the whole panel is energized and it's very easy to accidentally touch an energized part of the panel with a screwdriver or have a loose wire make contact as you are moving things around.

Be safe. If you have to ask this question, it means you need to turn the main breaker off.

Thanks for the advice. I kinda thought I should turn the main off to begin with, but the thought of not having to reset all the clocks, timers, computers, etc was nice.

A second question though... even with the main breaker turned off, is there anything I should watch for? Obviously, I shouldn't touch the two main leads coming into the main breaker, but is that it?

The only other issue is that if you move any double pole breakers, make sure that when you reinstall them that they are again across opposite hot bus bars.

I can't see how I can move a double pole breaker and NOT put it back across opposite hot bus bars? My double pole brakers are all double-wide and the slots in my panel are all A B A B from top to bottom, so is it possible to install it wrong? From what I can tell, it's alternating A B up and down while being the same A A left to right (i.e. A A on row 1, B B on row 2, etc.).

Isn't this standard? Or am I missing something?

The A/B bus is usually separated by a plastic tab. The plastic tab has to fit in between the 2 poles. If you put the double breaker in without the tab between the poles you'll only get 1 phase, not split phase.I don't know what kind of panel you have. Does it have plastic tabs between bus connections?

junkmailhold-
You should be safe if you flip the main breaker and stay away from the incoming service cables. I know it's a pain to reset the electronics, but I find I can work faster if I'm not constantly on guard with a live panel...and my blood pressure stays lower!

The only other tip I have for you is that you consider checking to see if all the setscrews on all the breakers are tight as they should be once you get through moving things around. Good luck.