How gradual is gradual?
On the first day of change, you should have plenty of the old dog food left. Add three quarters of their old food, and one quarter of the new food. You should do this for at least two days. After two days or so, add in another quarter, so you have half old food, half new food. Again, keep it at this level for another two days or so. Then add in a third quarter (one quarter old, three quarters new), and finally you can phase out the old food completely.
What happens if you don't do it gradually?
Refusal to Eat
On Presenting Your Dog With a Menu of Many Choices
I'm not talking about raw feeding here, and I'm not discussing home-cooked meals. This refers to a habit some dog owners have developed of having an array of dry dog food choices, or of buying whatever is on sale that week to the feed the dog and save money. Here's a little tip: It may save you a few dollars, and it may be convenient for you, but you are hurting your dog. Chronic indigestion, finicky eating habits, and chronic diarrhea are just three common things that will happen if you like to switch your dog's foods constantly.