Most cat owners are told that they should not make a sudden change to their cat's diet will result in refusal to eat the new diet or stomach upset.
Some owners will try to out wait their cat who is refusing to eat the new diet, but this is not a good idea as it can lead to a potentially fatal disorder known as hepatic lipidosis (or fatty liver disease) in which in response to the body not receiving enough nutrients, fat is sent to the liver to be used as fuel, unfortunately the liver is not very efficient at processing fat, which begins to build up in the liver, causing it to no longer function properly.
Why do I need to change my cat's diet?
He may have developed a food allergy.
The food you are feeding may no longer be available.
Switching over from a kitten food to an adult food, or an adult food to a senior food.
Changing from wet to dry, or homemade etc.
Bringing a new cat home, I always ask the shelter/breeder what my cat has been eating, so that I can have some of their regular food on hand and gradually transition the cat over to what we feed.
So how do I switch him over to a new diet?
It is always recommended to avoid your cat turning his nose up at the new diet or getting an upset stomach, that the switch is gradual, over a few days.
On day 1, add 10% new food to 90% old food, mix in well.
Day 2, add 20% new food to 80% old food, mix in well.
Gradually increase the amount of new food and decrease the amount of old food by 10%. If the cat stops eating the food as a result of the change, go back to the old food and try again, but slower, just adding 5% at a time.
You can also help to improve the palatability of wet food by warming it for 30 seconds in the microwave.
Feeding different types of food:
I will contradict myself a little here and possibly go against guidelines, but I have never been a fan of feeding one type of food over another (unless treating a medical condition). For example, feeding a wet diet only, or a dry diet only. I like to feed my cats a range of foods including canned, dry and some raw food. They are not finicky eaters (although one refuses to eat dry food), and I have found that they do tolerate changes to diet quite well because they haven't been given the chance to become accustomed to one "type" or brand of food.
I also think that there are advantages and disadvantages to all types of food. Canned is good because it is made up of 60-70% water, so even if your cat isn't a big water drinker, he is getting plenty of fluids, which can be advantageous if you have a cat prone to urinary problems (such as stones), dry food is good for their teeth and for them to graze on between their twice daily meal of canned food, I also like to give them cut up pieces of human grade beef which is good for jaw and dental health. Again, this is my opinion only but has served my cats well. I think balance is the key.