I have tried more than once to switch my two to raw but they just aren't interested and at 13 and 12 years of age I am not going to force it.
I have realized though after talking to holistic vets and doing a lot of reading that if possible cats should not be eating dry food at all. Mine have been on strictly canned for a few years now and it has made a HUGE difference in body condition and coat quality, they even seem happier. I was in the same position you were in with my one girl. She was 18 lbs and I was so worried about diabetes and her heart. After switching them to a grain free all canned diet and of course measuring the food appropriately for them (they do get slightly different amounts) my girl is a beautiful 12 lbs. She lost 1/3 of her body weight in 1 year and she is so much happier.
Do a little reading here
Feeding Your Cat: Know The Basis of Feline Nutrition to understand the rationale behind no dry food.