I will instruct clients to feed 3/4 of their regular food with 1/4 of the new for
about a week or so, then 1/2 and 1/2 for
a week, then 1/4 regular food to 3/4 new food for
a week then all new. This way,in the early transition, your kitty will primarily be receiving their regular diet so you can administer the insulin that is needed.
There are other dietary options to consider in the diabetic feline patient. Purina, Hill's and Royal Canin make a food that can be used in diabetes.