You do know they can replace the leadership right? If someone were to bomb the capital during the State of the Union (which is a plot to a TV show btw) and kill almost all of our leaders would the United States as we know it cease to exist? Would a non-democratic government take over?
Regime change has nothing to do with killing the current leadership. It's about changing the entire ideology of the government. Because if Iran stays a religious theocracy with a different group of Ayatollahs, what have you really accomplished?
To fully remove the Islamic regime in Iran you would need to militarily support the opposing regime. That means troops on the ground supporting them and fighting back against the Islamic regime loyalists. Unless the Iranian people remove the Ayatollahs themselves, it'll take a ground war from United States to change regimes.