One of my dogs had a very delicate stomach when I got her, so when we found a food that worked for her, we stuck with it. The problem was that it was one of the Diamond foods and they got recalled. So I had to do a cold turkey switch, which always really messed her up. A year and a half ago, just before the last major Diamond recall, I found this site and started hearing about rotational feeding and how it could help dogs with sensitive stomachs. I jumped on the band wagon and committed to rotational feeding. The first few times I switched, I took an entire month to do it. Then I cut it down to 2 weeks, then 1. Now, I switch food weekly with no transition over the summer, and before the weather got warm and humid I was feeding a different kibble at every meal. Once dogs get used to switching, they no longer need transition time or need only a very short transition. In the last year and a half, my dogs have only had soft stools once for a few hours and not had any real diarrhea at all. I have to assume that their gut is healthy and the microflora in their intestines are diverse. BTW, I live rural, so the dogs can find all kinds of disgusting things to eat, and they do, but they never get sick from them.
