Can I switch soy fed baby to breastmilk?

I want to re-lactate. My 7 month old is currently on similac soy, but I ve heard bad things in correlation to the development of the baby on soy. We also have a friend who is willing to sell her breastmilk. He was breastfed for a month, but I dried up, went on gentlease but was in some discomfort with it. Since he s been on soy, he s been a happy baby. He had no issue with my breastmilk at all.