I wouldn't use the Purina Indoor formula, it has too many dyes in it. Someone at our hospital accidentally got a bag of that like a year ago to feed our resident cats. They suddenly started acting all jumpy and no one could hold or pet them anymore. We finally figured out it had to be the food since there was nothing medically wrong with them. After about a week or two of being back on their normal food they went back to acting normally again. Turns out they'd had an allergic reaction to the dyes that showed up as a behavioural thing.

I've also seen a TON of cases come in our doors with all kinds of skin/ear problems from dyes in foods. If you're going to feed Purina stick with the plain cat chow formula, or just stay away from the dyes.

Switching foods around is not a good idea. Find one good food and just stick with it.