How do you transition from one food to another? My 9 wk old English bullmastiff has pudding-like stool. I am switching from purina large breed puppychow to iams adult fish. Vet said 50/50 but I don't know how long to keep it at 50/50. Also after research, I am going to Eagle pack large breed puppychow which should be here any day. Now I need to know how to transition to Eagle. My pup is not making formed stool yet. He also is scratching his ears and biting his legs. This is my first bullmastiff.