Any introduction to new food has to be done gradually. Here's what I did when I switched Max and Speckles to a low-protein diet.
Week 1
80% old food and 20% new food

Week 2
70% old food and 30% new food

Week 3
50% old food and 50% new food

Week 4
30% old food and 70% new food

Week 5
10% old food and 90% new food

Week 6
100% new food

If you don't do it gradually, it can cause tummy upset and the runs.

Good luck.