You want the revolution to start in your capital. If it happens anywhere else, it's just a revolt that has to be squashed. Any armies in the capital will join the "old guard," so you might want to move them.

It seems that if you want Constitutional Monarchy, you have to cause the lower classes to revolt, converting to a Republic, then tick the upper class off, which will make you a Constitutional Monarchy. In Absolute Monarchy, making the upper classes angry doesn't seem to change your system of government, it just ousts your current king.

If I have a troublesome lower class will establishing a constitutional monarchy still make them happy?
It won't make them happy but it does remove the negative happiness modifer from Absolute Monarchy, IIRC. This is all from memory but I believe Absolute Monarchies have higher repression but it causes unrest with the lower classes, plus it hampers research and your economy.