You could speak to your vets about the R/D diet from Hills, which you can then change to M/D once they have the weight off or the Calorie Support diet from Royal Canin.

You need to leave this food down that you are changing to mixed with the old food for a couple of days and then remove the old food. When the new food is down and they don't eat it. Do not fuss and do not give them another option. Leave it and they will eat it eventually. Cat's are very cheeky and will work out that you will give in if you have done previously. BE STRONG!

Also you need to think about what you are feeding now and how much you are feeding. Your vet or one of the vet techs should be able to walk you through a healthy weight loss program and help you work out what measure of food is best to feed.