It can vary from dog to dog. Some can make a switch with no problems at all while others can get some diarrhea.
If your switching from a grain inclusive kibble to a grain free kibble I would take it pretty slow.
I would start off with only 1/4 of the food being the new food for a few days to a week. If after a few days her stool looks good up it to 1/2 of the food being new food...and so on.
Lucky gets 1 cup in the AM and 1 cup in the PM. When I switched her over to a grain free for the first week or so I did 1/4 cup new food, 3/4 cup old food. Then I uped it to 1/2 and 1/2 for about a week. They 3/4 new food, 1/4 old food for a few days...etc. You get the picture. Lucky did great, never got diarrhea. However, she did have gas for a couple days....:biggrin: