Typically, you shouldn't see actual side effects when switching your kitty from hard or dry cat food to soft or canned food. The significantly higher moisture content in wet food can cause some changes, though. Still, the switch requires consideration of some main differences between the two types of food.
Your four-legged friend may pee more when you switch from dry to soft food. This is the most likely side effect. Hard food has only 5 to 10 percent moisture, but canned food is about 78 percent water. That's a lot more fluids for a little animal like your kitty, and the discrepancy is particularly significant for a kitten or small individual. Your furry buddy may drink less, too, since she's getting more water from meals. Peeing more isn't necessarily a bad thing, though; water's an important nutrient, and increased intake helps prevent dehydration and it's good for critters with diabetes, kidney or urinary tract problems and other conditions.
Lots of kitties are picky eaters, and feeding canned food exclusively can make the problem worse. Most felines find wet food to be much yummier than kibble. If you switch your kitty to the good stuff and she gets to indulge her palate in it every day, she may become less and less interested in foods she deems inferior. If you're committed to the switch, this may not matter. Keep in mind, though, that sometime down the line you may need or want to change foods again, and your kitty might be more resistant than you like.
Switching your fuzzy friend from hard to soft food might result in weight loss, a positive side effect if she's overweight. Because of the high moisture content, canned food is more filling than dry with far fewer calories. Also, dry food is usually high in carbohydrates, which isn't particularly important to cats; they're obligate carnivores who get energy mostly from protein and fats, while carbs are readily stored as fat. Wet food is mostly animal-based protein and fats, which gives your kitty lots of energy, so she'll probably become more active, too, which also promotes a healthy weight.
If you notice side effects like rash, hives, itching, redness, facial swelling, upset stomach, diarrhea, vomiting, coughing or sneezing after switching your kitty from dry to canned food, you're probably dealing with a food allergy or intolerance to something in the new product. Gastrointestinal symptoms are typical with an intolerance, while allergies can present with those and/or the others mentioned. Your kitty may have a problem with poultry, beef, pork, fish or another ingredient commonly found in wet food. See what's in the new stuff that wasn't in the old stuff, then consult your vet about designing an exclusion diet to identify the offending ingredient.
Eric Mohrman has been a freelance writer since 2007, focusing on travel, food and lifestyle stories. His creative writing is also widely published. He lives in Orlando, Florida.