It may be normal to child having change in stool frequency and consistency after change in diet. I want ask you some questions! How you offer the feed to your child, I mean by bottle or by katori/spoon?? I advised you to use katori spoon if not using... Also start other house hold food items also apart form the formula like cooked daal, cooked rice khichdi, curd, cooked vegetables, fruits, halwa, raab etc...

Kindly consult your pediatrician regarding this. It is unlikely if you are using the same brand as earlier. There may be other causes for the diarrhea which your pediatrician may help recognise