Oh dear, half a can is definitely too much to start with, for dry fed cats. I'm sorry I wasn't more specific.
Cats do need food changes to be done very very slowly.
A teaspoon of canned, once a day, would be where to start. Not mixed with the dry. Just give them each a teaspoon of canned by itself. A teaspoon a day for a week, and of course the probiotic mixed in. Then the next week, two teaspoons a day, say one in the morning and one at night. The next week three teaspoons. And so on. The probiotic is very important.
Food transitions are never done quickly. They take a long time, and yes, some trial and error. Remember that if friskies doesn't agree with them there are plenty of other choices. And even with friskies, one formula might not work, when another friskies formula will.
It can be done. And it is so so worth it. Many of us have done food transitions with our cats, either from dry to wet or canned to raw, or even dry to raw.
It can be done. A few months of effort now can save you hundreds in vet bills, and so much stress for your cats, in the future. Especially with male cats. Cats are so dehydrated on dry diets, their urine is so concentrated, urinary tract problems are rampant. And the male cat has very narrow urethra. The danger of blockage is very high. In another forum I post in there is a thread on Perenial Urethrostomy that is constantly being updated with new members looking for advice because their male cat has blocked and the vet is advising the PU surgery. All are or were fed dry diets. Most of them, their vets never even ask about diet, except to recommend some "prescription" diet, usually dry.
But, back on topic with the pooping out of the box, I do agree with the member who suggested that another de-worming may need to be done. Often cats blame the litter box for discomfort and so try pooping else where. Anyone, including a cat, who is pooping 3 times a day has got to be experiencing some discomfort.
But hang in there with the canned food, just start with much much less. Wait until you get some probiotic if you want, before starting again.Edited by otto - 10/25/12 at 7:14pm