Is it normal that after changing milk, the baby makes a rare poop after each feeding?

Rare stools can be the result of an incorrect way of changing milk - you do not describe how you did it, but I conclude that right away. The new preparation should be introduced gradually. Suppose that so far you have used 3 teaspoons of Nutramigen powder to make the Nutramigen mixture. When changing milk to Bebilon Pepti Syneo, on the first day you should make a mixture of 1 teaspoon of Bebilon and 2 teaspoons of Nutramigen, the second - 2 teaspoons of Bebilon and 1 teaspoon of Nutramigen and only on the 3rd day completely change the milk. Of course, this is an example - I do not know how many teaspoons you use, but every day you should introduce 1 teaspoon more of a new preparation.