I think it just depends on the puppy. Some dogs can tolerate food switches very easily. I dogsat for a 5 month old pup for the past 10 days who was eating Beneful and switched cold turkey to Acana perfectly fine, in fact the improvements made within that short timespan were awesome.
But then some pups will get major cannon butt when doing it, so I guess it's best to be on the safe side if you have a new pup with an unknown digestive tract.
Once you get to know your dog better, you can figure out how fast they can switch, etc.
My personal favorites are Fromm and Acana.