My little girl Tikka is stubborn to eat the food I'm switching her to, so I actually made a little game out of it... I take maybe 20 pieces of food in my hand, 10 of her old food 10 of the new food and I throw them one at a time and say "go get it!" and she eats them like she doesn't even know the difference, then after those pieces are gone she'll go to her bowl and eat the rest until she's full.
just an idea
otherwise she just picks through her food and leaves all of the new kind of food in the bowl.