No, that's not actually what you said. You said If the box (main panel or disconnect box) still has power going to it, then it's possible to shock yourself or worse by changing out a breaker. If the breaker is in the main panel, then the main breaker is what takes care of that. If it's in the disconnect box, then the breaker in the main panel will suffice. Just because the breaker has no power coming out of it, does not mean you cannot get shocked, or worse changing out a breaker.