We often see advice to introduce new dog food slowly, so that the dogs get used to it, but our experience is that it makes no difference at all. Our dogs, and our friends dogs, favorite food is ODF. When they go into someone else's house, or visa versa, they first thing they do is check out the ODF and gobble it down. I've never seen any ill effects from this. Also on the few occasions when have had to feed a different kibble I've never seen a problem, even though it was a complete change all at once. Does anyone else have dogs that have problems changing from one food to another? I'm really only thinking about high quality kibble, not real cheap junk. We have put dogs on roast chicken and rice a few times and again never had any issues, other than solving what ever problem caused us to go to chicken and rice.
I have changed Juno's kibble several times, on vet advice, and each change has been phased over a week to ensure that there is no reaction. When she has an upset and been on a diet of fish and rice I again phase her kibble back in over several days. I've never had a problem with her eating other dog food when we visit a friend or family as none of us leave food down. They receive their ration for breakfast or dinner, anything uneaten in 15 minutes would be removed and disposed of.
Our friends Sammys are both free feeders, so they often have food down. Before we got Cooper, Tilly was a free feeder, but we can't leave food down with Cooper around. Either she doesn't have the POMC gene, or she is just a greedy growing puppy.
I've changed Maisie's food a number of times now, with no phased change and there has been no ill effects. She eats ODF if she has the chance!
I found Oban could change cold turkey when the food I was reserving for slow mixing was all fed to him and it was Sunday night so starve or cold switch and he had no problems. I rotated three kibbles, different brands, different main protein and filler sources, one grain free and he could cold switch. OCF goes down equally as well. So did horse poop, dead frogs and bullheads (fish heads).
When we brought Willow home, we forgot to bring the bag of her food from the breeder. Since that was a four-hour drive, and we couldn't get what she was on locally, we changed in one go. She didn't have any adverse effects. I've just changed her from Burns Original to Burns Sensitive+ over two days and didn't have any issues with her poop at all. Shadow, however, is a lot more sensitive to changes in his diet and will always get runny when introducing a new food for the first time.