For the most part all of ours took right too it also. Kiwi was the only exception. For her I mixed it the natural in with her fruit flavored. Slowly at first and just kept increasing the amount of natural. I did this over a couple of weeks at the end of a bag of the fruit flavored. When I was out of the fruit flavored she was ready for the natural.
Maybe it helps that I keep the natural in an old plastic jar that the fruit blend used to come in. Remember those? We kept one after they switched to the bags to put the bags in. I like the screw top lids better than the zipper top bags any day.