It is important that your dog switch dog food gradually from his current dog food to avoid digestive upset or stomach trouble like diarrhea and vomiting.

You have to remember these facts when changing your dog’s food:

Puppies become adults at 12 months of age and should transition to an adult dog food to ensure they are receiving proper nutrient levels for adult dogs.
Large breed puppies and small breed puppies should switch to a large breed or small breed adult dog food to ensure that their special needs are met.
For small and medium size dogs who are older, about the age of 7, they should transition to a mature adult or senior dog food that ensures that they are receiving the appropriate level of nutrients for that older life stage.
For large breed dogs that are around 5 years of age, their food should switch to a mature adult or senior large breed dog food so that their special nutrient requirements are met.
Pregnant or nursing dogs need energy-dense foods with increased calcium content so be sure to transition them during this special time to a puppy food. However, during pregnancy or nursing, large breed dogs should be switched to a regular puppy food, not a large breed puppy food.
If your veterinarian has recommended a therapeutic dog food for a specific health condition, please be sure to discuss transitioning his dog food in detail. There could be some special considerations and suggestions to ensure success.

Method for switching food for your dog

To transition, mix your dog’s current food with his new food. Over 7 days, gradually decrease the amount of the current dog food while increasing the amount of new dog food.

In order to avoid this problem change to the new food gradually. Get the new brand while still keeping some of the old brand.

Mix one part of the new food with three parts of the old food for at least two days.
Then observe the stool of your dog, if it does not have watery or loose stools, then mix equal amount of the foods for at least two days.
If he is still dry without having any loose stools, then mix three parts of the new food with one part of the old food for at least two days.
If the stool is still normal, give him the new food wholly for a few days.
If he has any loose stools during the switch from one food to another, revert to the ration in which the stool is solid and normal.
There may be some reactions during this change like losing some of his coat, this is normal because the fur grows back within some few months.

If the diarrhea ensues at any time consult your vet. You can stop the diarrhea at times by withholding food and water for twenty four hours
Source:http://petsarena.com.ng/2017/05/best-way-to-switch-food-for-your-dog/