You will know why a food change is spread out, what a smooth switch looks like, and when a soft stool stops being normal.
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The old bag is empty, the new one is bought, and the temptation is to just pour. What happens next shows up in the litter tray, not in the bowl.
Gut flora settles around what it has been given for weeks. Changing the raw material from one day to the next asks it to reorganise fast, and some animals manage without showing a thing.
Others answer with soft stools, wind, sometimes a refusal to eat. Nothing tells you in advance which group yours belongs to, and one easy switch does not guarantee the next one.
The principle fits in one sentence: replace a growing share of the old food with the new one, a little more each day, until only the new food is served. A week comes up often because it gives digestion time to keep up.
What matters more than the number of days is the stop rule. If stools get worse, you hold yesterday's mix for another day instead of moving on. The pace follows the animal, not the diary.
In the first days, a stool looser than usual is an expected answer. It often settles by going back to the previous proportion.
What is a different matter: repeated liquid stools, blood, vomiting alongside, a flat animal that stops drinking. Duration counts too, and it counts far faster in a puppy, a kitten or an older animal.
Alternating one meal of old food and one of new looks like a transition, but the gut receives two different foods a few hours apart. That is two changes a day, not a progression.
Mixing in the same bowl keeps every meal the same composition. It has a second benefit: a picky animal finds it harder to pick out the bits it likes.
A switch started the week of a house move, a boarding stay or a new animal arriving becomes unreadable. If the animal refuses or digests badly, nothing tells you which of the two is to blame.
When you can, move it later. When you cannot, keep the old food through the event and change afterwards, once the house is ordinary again.
Many cats take badly to anything new in the bowl, especially when they have eaten the same thing for a long time. A cat that met few textures and few flavours young often stays choosy as an adult.
The practical consequence: with a cat you move in smaller steps, and you watch the total amount eaten above all. A cat that stops eating for several days can develop liver trouble, which makes a long refusal far more urgent than in a dog.
An older animal, one just over a digestive episode, a dog that already took a change badly, a cat that already turned a food down: in those cases the switch is spread over weeks rather than days.
Going slow costs nothing beyond keeping some of the old bag aside. That is why you buy the new food before the old one runs out, not on the day the bowl is empty.
If the animal refuses outright, or if digestion does not settle, returning to the old food is a normal decision. You are back on known ground, and you can start again more gently or try another route.
What you keep from the attempt: the date, the food, what you saw. That is what makes a conversation with a vet useful, and what stops you trying the exact same thing again six months later.