Didn't think we had a thread for this, but sometimes I hear of worried people who's dog is having diarrhea or vomiting and they don't know that its their switching of foods that caused it.
Usually when a dog eats their food their internal system has a way to adapt to the food, beneficial microbes in the GI track balance itself out for optimal digestion. Now what happens when you take that food away and all of a sudden give them a new one that might have different nutritional values or different ingredients? Well understandably the dog may go through a period of time where he has an upset stomach along with some diarrhea or vomiting.
You can simply allow the dog some time to adjust to the food or you can prevent it all together.
When switching foods you always want to switch it gradually. Here's an idea of how you can do it:
First day: 25% new food + 75% old food
Second day: 50% new food + 50% old food
Third day: 75% new food + 25% old food.
Fourth day: 100% new food
Now this is a transition over 4 days, if you want to (if your dog has a sensitive stomach) you may want to take it slowly and add even less of the new food. Then again there are some dogs that may not react negatively with the switches!