Ruth, I was just trying to cover all bases, since I did not know the whole story. I would then call your vet and tell him what you are doing and have him advise the best possible way to do this.
You can also start increasing your cat's activity by using the cat condo if you have one, and feeding her on the top level. This way, she has to work to get her food. You can also start hiding catnip treats around the house it high places where she has to work to get to them safely. The vet advised you to use canned food (or this is my best gussestimate anyway) because when well-intentioned cat owners feed dry- they always tend to overfeed. My vet has a favorite saying that I picked up long ago. You want to feed the cat, not the bowl. Feeding canned food, people tend to not put so much of it out. Also some cats tend to gain more weight,because they gulp their food (a genetic trait handed down from ancestors trying to survive) You can elevate the food bowl to slow this type of action down. Good luck, and yes, I would use RC products, they really do give good and lasting results.