Milk Concern

Hi mga mommies, question po..dapat po ba yung milk ni baby is recommended ng pedia nya or pwede pong magpalit ng milk even if walang recommendation si pedia. My baby's milk po kasi is similac gain plus pero napansin po namin na medyo constipated cya. We asked her pedia if we could change milk, kaya lang po di pumayag. Just want to know your thoughts po on this. Thank you.