I've always slowly switched foods when I decide to change what I am feeding, but mainly because I wanted to use up what was left of the old food
I know some people just switch "cold turkey" without any problems. There have been times when the store where I purchase my food is out of the kind that I am currently feeding so I will buy a bag of another kind that I had fed and know that my dogs did not have a problem with. But if it is something completely "new" I make the change slowly to see how they do with it.
As far as why people rotate foods, I'm not sure about anyone else but for me it's just so that I am offering a variety. I know I wouldn't want to eat the same thing day after day for the rest of my life
