if your gonna switch try a quality food
you will get better results since these foods are more protein based that grain based.

Eagle Pack Holistic/Regular/Hairball all are great
Sensible Choice
Fromms
Excel
Nutro
California Natural
Back to Basics
Felidae
Solid Gold
Artemis
Pinnacle

these are top of the line cat food, they use quality meat and less grain, hence LESS Weight gain.

Iams, Science Diet (sorry no offence) use more grain than protein.
purina, friskies, meow mix etc anything you can buy at Walmart or a grocery store are worse & pretty much 90% grains, which adds no nutritional value to your cat. Eukanuba which is made by Iams the same. The food listed above are catorgized top of the line, some you may have never heard of because they don't advertise, their money goes into their product not marketing.

They may not be mulit-millionare companies but their pet food is the best. Stick with those and your cat will do great. I don't push any particular one, they are all good.