We got our cat a week ago and she came to us eating Whiskas kitten and Royal Canin kitten food. The breeder told me what brand she used but didn’t specify kitten. She’s 18mo so I bought the grown up versions as she’s been over 12mnths. The breeder gave us a few days of food and I stupidly didn’t do a transition period when we ran out as I thought with them being the same brand she would be fine.
She is not fine 
She’s had the stinkiest soft poos since Saturday, she went twice on Saturday, about 5 times yesterday and at least once last night. I’m withholding food for 24 hours, so she’ll be eating tonight, and I’m feeding her plain boiled chicken til her stomach settles.
However, I wanted to transition her off of Whiskas and onto Bozita and I’m not sure whether to go back to the old kitten food and use that to transition, or if to ‘reset’ her with the plain chicken til she’s better and put her on the new food.
I’ve never had such a sensitive cat, all the ones I had growing up got fed whatever was on offer at the supermarket 