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...