A cat's tummy gets used to the food it eats every day. Change it overnight and you can get an upset stomach, and many cats simply refuse the new bowl. PDSA and the Cat Care Center both advise changing food gradually, over at least a week and often two. PDSA adds: never change it suddenly unless your vet tells you to.
New food on a separate plate, a little more each step, and go back a step if your cat says no.
Pick how your cat usually reacts to change. The plan shows what share of each meal to offer as the new food, day by day.
PDSA describes a switch of roughly two weeks in three stages. At first, give the usual meal plus a small teaspoonful of the new food on a separate plate. Move on only when your cat has eaten it happily for a few days. Then slowly increase the new food and reduce the old, still on separate plates. Once your cat has eaten both foods reliably for about a week, offer only the new one. The Cat Care Center adds that some cats need up to three months, so patience beats pressure.
Ask a vet before changing food for a cat that is ill or on a prescription diet, or if you are unsure which food suits them. Cats Protection says to call a vet if your cat has not eaten for over 24 hours, or not drunk for 24 hours (for kittens, sooner: 12 to 24 hours without food). Also call if you see repeated vomiting, diarrhoea or unusual tiredness. Do not try to treat these yourself.