When changing a cat diet you must go slowly.
A sudden change of diet can upset digestion and even make her sick. If this happens, it is unlikely she will ever change to the cat food she needs.
And cats don't like change. If you just put out a new food, they may refuse to eat it simply because it's new.
To change a cat diet, mix a small amount of the new food in with the old food. Feed the mixture to the cat for 2-3 days. Continue mixing in more of the new food while decreasing the amount of the old food every few days. Do this over a 2-3 week period until the cat is eating 100% of the new diet without problems.
During the transition, if he stops eating the current mixture, go back to the mixed amounts he was eating before he stopped. If necessary, start over and go slower - smaller amounts of new food added over a longer period of time.
Keep in mind that cats prefer food at room temperature or slightly warmer. If your cat won't eat his new cat food, try warming the food a little to release the aroma. A cat's appetite is stimulated by smell much more than taste. You can also try dripping a bit of tuna juice over the food to improve the smell.