Changing foods for most healthy dogs isn't really a big deal. In fact, some owners switch their pets from one food to on a daily basis. Most dogs tolerate these changes.
However, vomiting, diarrhoea, burping, farting or a combination of signs may occur with sudden, rapid switching of foods, probably because of ingredient differences.
To be on the safe side, it's a good idea to change your dog's food over the course of at least three days. A seven-day period is even better. We suggest increasing the proportion of new food and decreasing the proportion of old food as outlined in the table below.
While nearly all dogs readily tolerate a seven-day transition period, a much longer transitional period is recommended if the food change is significant, the dog has demonstrated a poor tolerance to such changes in the past or food refusal is expected.
(From Hand, Michael S., Craig D. Thatcher, Rebecca L. Remillard, Philip Roudebush, Bruce J. Novtony, and Lon D. Lewis. 2010. Small animal clinical nutrition. http://www.markmorrisinstitute.org/sacn5_chapters.html.)