How to change your baby's milk

First, my baby used Hypoallergenic milk as I thought he had allergic as he had some red spots on his skin since he was born. Yesterday someone called me to tell me about milk product I didn't use. I thought she got my cell phone number from the hospital where I performed Caesarian Section. She said that to change my baby's milk, first I needed to mix the old milk with the new milk. First I need to use more old milk than the new milk, then I can use less old milk and use more new milk. At the end, I can use the new milk without using the old milk anymore. Yesterday my husband bought new milk. The milk was not Hypoallergenic milk as I wanted my baby to use regular milk because the regular milk was easier to buy and I was worried if I continued to use Hypoallergenic milk my baby would have bad immune. Luckily, my baby had no problem after I changed the milk.