Replace Circuit Breaker on Split Bus Panel
6 of one, half dozen of the other on the red or blue arrow issue. You can plug a breaker in hot (assuming you know the proper safety precautions). I would probably swap out the unused one, just because the panel cover would not need to be altered (I'm lazy).

If you are uncomfortable working on live equipment, you can ask the utility to disconnect you (pull the meter). Generally that means they come out and do it one day, then come back and put it back in the next day. SOMETIMES you get lucky and they do it faster, but you can't count on it, so be prepared to have no power for a day. If you have one of the new "Smart Meters", they often have a remote disconnect capability so the utility simply sends it a signal to disconnect or reconnect, so it can be just minutes.