The Wellness Core is a very good food but you mentioned a sensitive stomach. I am not sure if that food will be too rich but you can certainly try it. I might look for something that is not as high in protein, similar protein content wise to what you use now. I would also look for a food that uses a single meat or fish as its protein instead of one that uses a combination.
Naturapet California Natural Chicken Meal & Rice http://www.dogfoodanalysis.com/dog_food_reviews/showproduct.php?product=433&cat=4Fromm Family Foods Fromm Four Star chicken a la veg (all-life-stages)http://www.dogfoodanalysis.com/dog_food_reviews/showproduct.php?product=8&cat=3Dick Van Patten's Natural Balance Organic Formula Dry Dog Foodhttp://www.dogfoodanalysis.com/dog_food_reviews/showproduct.php?product=168&cat=4Dog food analysis seems to like California Natural for sensitive stomachs. I noticed your Petco carries DVP's Natural Balance.