One of the reasons that I rotate the food is that then you really do not have to change slowly or at least that is what I find. I change every three months to a different food going between Wellness, California Natural, EVO, Fromms, etc… you get the picture. That way they are used to constant change (also adding in veggies and fruits both raw and steamed along with yams, etc).
If I am on the road and need food for the B's then I never have to worry that I will not be able to find what I am feeding them at present... as they are used to change...