I went from a standard double to a triple and now want to go to a compact double. I don't have much knowledge with Shimano but it was easy with Campy. With Campy, I didn't need to change the shifters because Campy uses the same shifters for both doubles and triples for 10 speed at least. All I needed to do is change the crank and front dr.

The advantage of a compact for me isn't because it weighs less or shifts better but because the gearing is more useful for me.