what i've always done with all the dogs we've ever owned, is to take what they're eating currently, and mix it half and half with the new stuff for a week... then gradually take them off of it. just got my lab saturday, and the SPCA feeds all the animals Science Diet, so I got Iams and mix them together to gradually but surely take her off Science Diet.
I too feel that if a dog food works great for your dog to just stick with it instead of changing foods often. There are so many food brands out there --some better than others. Each dog does differently to certain foods. Everyone has their own opinion on which food brand they think is the best. What matters the most is what works well for each dog.
I feed raw and my guys get something different every week. I see no problems with them eating different food, as long as they can handle it. I think GI problems comes from the fact that most dogs aren't used to switching foods. I have a friend from when I was a kid till I graduated university, and all he eats are PB sandwiches, chicken, tuna and burgers. He's perfectly healthy eating limited variety of food, while I on the other hand eats everything, so I like my dog to be able to do the same thing also.
I've switched food on my girls several times over the past 8 months. I haven't done much, just start the new bag. I changed most recently because of an apparent allergy. There is no way I'm going to mix in the old food with the new. Poor baby has enough raw spots from chewing. I haven't had any problems with GI upset. However, GI upset is the reason I started changing foods in the first place.
I change dogs food often as well depending on the season. During the summer I feed Canidae and during the winter I feed Evo. I never do it slow, mix the two or anything. I just switch them. Never had a problem.