Hi J. -
When I weaned my now almost two-yeat-old son off of breastmilk, I would supplement with soymilk. I don't care for cow's milk, for health and other reasons, so soy was all I consumed both while I was pregnant with him and during breastfeeding. My husband, however, still likes to drink cow's milk (2%), so from time to time we'd let our son try it. Well, the cow's milk messed up his little tummy so bad! Ugh! Come to find out, he was lactose intolerant. He could handle all the other dairy products I ate while pregnant/breastfeeding, like yogurt, cheese, etc., but he couldn't handle cow's milk. We've since been working with him, offering it to him repeatedly in small quantities, and even though soy is his first choice, he can tolerate cow's milk much better now. I think that introducing it to him at such an early age (about a year old) helped him become accustomed to it, thankfully, so I don't have to worry when he consumes cow's milk now.
Just for shiggles, since we're now pregnant with baby #2, I've made it a point to drink a little cow's milk ever so often, so the baby can get used to it in utero. Hopefully that will save us from having to go through the same thing with them as we did with our first child.
Hope this helps, and good luck!
PS--> I was worried about what I had heard about the effects soy might have on my son, being a male and all...But do you know how much soy a male has to consume in order to experience negative side effects? Let's just say, a LOT. Research it. :) You know back in the day there was a scare regarding lavender and negative side effects on males...Same thing. They would have to be exposed to a lot of lavender to experience the effects. I mean, if male vegetarians can consume all the soymilk and tofu and edamame they want and not grow breasts and have emotional problems, I seriously doubt drinking a glass of soymilk a day would do any harm. Just make sure your soy product is non-GMO certified, meaning the soy was grown w/o the use of genetic modification or engineering.
(I was just poking fun at male vegetarians, but seroiusly, soy is okay.)