Not only doesn't replacing Mousavi with Ahmadinejad replace the mullahs -- it may even entrench them. According to the India Times article posted earlier in this thread, the Supreme Leader is not necessarily the preferred "kingmaker" in Iran. Ayatollah Khameini's mainstay of support (and Khameini is Ahmadinejad's backer) is amongst the rural areas and, most importantly, the Revolutionary Guard and the armed forces.
The mullahs prefer Rafasnjani (who backs Mousavi). Rafsanjani leads a coalition of, primarily, the religious establishment and the business leaders.
What this election really represented was a fight between the army and the civilian elites. Khameini may be Ayatollah, but that doesn't make him pope of Iran. A more apt analogy may be Emperor Claudius of Rome, who was unpopular with the nobles of which he was nominally the highest, and kept power through common touch and a close relationship with the Praetorian Guard. After Claudius passed, however, the Praetorian Guard basically ran Rome for 15 years until Vespasian finally neutered them.
Another historical analogy could be the movement from Lenin's Soviet Russia to Stalin's. Lenin's USSR was run by men who were with him during the Russian Revolution, including Stalin. Stalin consolidated true power during World War II, and for the next fifty years, only WW II vets could control the State. Khomeini surrounded himself with men from the Iranian Revolution, and ultimately chose Khameini as his successor. But Khameini basically surrounds himself with people whose political maturity occurred during the Iraq/Iran War, marginalizing the original revolutionaries (like Rafsanjani and Mousavi). Stalin managed to exile or purge his Leninist comrades. Khameini wasn't so able.
What does this mean? First, this isn't about democracy, except insofar as neither side wants another popular uprising, and the election was supposed to mollify the masses, not empower them. Second, the fight is really a struggle between the military and clericy/bazaar. The military currently has the upper hand.
What does this mean for the West? Well, nothing good. It is often said that you're always fighting the last war. Khamieini/Ahmadinejad are fighting the Iran-Iraq War. They see themselves threatened by military invasions and strikes from the US (which has occupied two of its neighbors), Saudi Arabia and Israel. Rafasnjani/Mousavi are sill fighting the revolution, and see themselves threatened by Imperial design and philosophical disaster. They're the faction in the government pushing Hezbollah funding (created by one of Mousavi's proteges).
So which Iran do you want? A paranoid xenophobic military dictatorship yearning for a nuclear deterrent and willing to oppress its people with overwhelming brutal force, or a terror-sponsoring fundamentalist regime that wants to expand its revolution into Afghanistan and Iraq (and likely willing to oppress its people with overwhelming brutal force)?