Girls, tell me about transferring from mixture to mixture. How should …

Girls, tell me about transferring from mixture to mixture. How should I do it correctly? How long does it take? In one hospital my son was eating Nutrilon, now transferred to another and they give him Niustozhen, but can you immediately switch to another after one mixture? His tummy hurts every day, like a stone. I massage him, put a warm blanket on his belly, but he still screams wildly(( How can I help him?