I use "all life stages" foods, so no need to switch. I tried puppy food, but my senior would get into the puppy food and the puppy would get into the senior food, so ALS solved that issue.
I have never had luck with gradual transition, either. They tend to eat only the new stuff, leaving the old kibble spread out all over the floor.
Luckily I've never seen a digestive upset from a quick transition, but they don't get any "junk" dog food, so I guess that helps.