Any kind of chemical doesn't help. The animals' bodies get plenty of toxins from their environment. We should try to limit the amount that is added in the food. I can't stress it enough. Pets may react to their new diets by getting sick at first, but often these symptoms represent old ailments that were never fully cured when treated with allopathic methods.
Digestive difficulties are fairly common, as is the passing of worms.
Sometimes, old skin rashes will re-surface. If your pet's overall energy level is high, then these symptoms will probably subside in a few days. However, if your pet's energy is low or there is a distinct change in mood, seek veterinary help. Be on the lookout for food allergies. Years of packaged food may have given your pet multiple food sensitivities.