Why would the West have to support such a change? It can only happen from within, just like that last time that Iran decided on Regime Change and showed they were perfectly capable of getting rid of their leader of the day (the Shah). Persia ceased to exist and Iran was born, much against the wish of the West and without any of its support (rather the opposite). I'm pretty sure that Iran can manage the next transition just fine. Though it may be a while. If we want to expand IS territory then by all means, let's mess in Irans internal affairs.