Any change of food should be done over a week at least, longer if the dog has known digestive problems ie: colitis. If you are confident that the diahrroah is food related, starving her for 24 hours, and only giving her water, and gradually introducing a bland diet of chicken/fish and rice should resolve it, and then introduce the diet you want to feed her with.
Acidophullus tablets which replace and maintain the gut flora in the stomach are available from any health food shop and some chemists can be added to the drinking water or food.