How long has she been on the canned food? Sometimes when cats first switch, it can take a week or so for their systems to adjust to the food. Fortiflora (a cat probiotic) can help a lot with these problems. You can get it from your vet, or
online where it's way cheaper.
Even if you change the diet slowly (over a week) for some cats, they still can have GI issues because of the change. These will resolve on their own and are no reason to change the diet back--doing that will just have to make you start over again. Add some water to her canned food to make sure she's getting enough moisture to prevent dehydration.
What flavors of Fancy Feast are you feeding? What dry food was she eating before you switched her? If the runny poo doesn't resolve on it's own in a week or so with the probiotic, then it's possible your cat has a food intolerance to one of the ingredients in the Fancy Feast. The most common protein food intolerances for cats are Fish, Beef, and Chicken, in that order. If we compare the ingredients of the dry she was eating to the Fancy Feast ingredients, it might be revealing as to what the problem may be, if it's not just a system upset because of the change in diet. If it is a food intolerance, all you have to do is avoid the offending ingredient and the problem will resolve.
High fat content can also cause these issues, but Fancy Feast isn't super high in fat so I wouldn't guess that's your problem.
There is no dry food that is good for a diabetic cat. Even the lower carb dry foods still raise blood sugar levels in many diabetic cats. Bandit's blood sugar will shoot up by about 100 points from even the low carb dry food. Even if the cat is one that can tolerate it, you are still setting yourself up for future Urinary, GI, and kidney problems with it.