I've heard some very good things about adding canned pumpkin to their diets, just a tablespoon can help with diarrhea or constipation. I think I would try that first, and see if it helps. Remember its canned pumpkin, not the pie filling which has other ingredients. You might want to read a little more about dog foods, we have a diet forum, and this site has some great advice on dog foods.
The Dog Food Project - Identifying better products If you decide to switch, have a plan in mind, do it very gradually, dogs do not like sudden changes in their diets and may experience vomiting and/or diarrhea if the change is made too suddenly.