Mine have never been "cooperative" when transitioning, and it never caused problems. Yes, some (maybe even most, I don't know) dogs will have a problem if they make a sudden switch, so it is good to take the precaution to transition slowly. But, like I have seen with my own dogs, you might not see any problem at all if they only want to eat the new stuff and leave the old stuff behind.
__________________ Life is merrier with a Yorkshire Terrier! Jezebel  & Chuy ... RIP: Barkley  Loosie & Sassy  |