There is old saying that says that changing food on a dog can make it sick to its stomach. And that if you want to change the dog's food, you need to slow introduce it by mixing it in increasing percentages with the old food until he is eating only the new food.
However, in my experience, this doesn't seem to be the case. My wife's dog used to get whatever was on sale when he was a puppy and never showed any problems. His diets has since stabilized so his dry food doesn't change much, but we do switch the wet food every now and then based what's on sale (he only gets a spoonful or 2 of wet food with every meal so it diet is mostly dry food). All of the other dogs I've had have not shown any sign of problems after an abrupt diet change.
My parents, however, always subscribed to this idea, so they religiously get the same food all of the time and in the rare cases the switch, slowly introduce it over a several week period.
So my question is there any truth to this idea or is it a old wives tail?