We tried transitioning to milk and DD wouldn't do it. Tried all different types of milk and she just would not drink it. The Dr said she didn't have to have milk as long as she gets the calcium and vitamins from other sources. At 22 months we are now starting to try milk again but it's still very hit or miss.
We actually gave regular formula a go again when he was about 10 months. We transitioned from soy to regular over a week and he did fine with that surprisingly. So when it came time to transitioning to WCM we did it over a week as well and he did well with that as well.
My DD was on soy formula and we transitioned her to soy milk, just by gradually adding more milk/less formula over a period of a couple of weeks. No problems with her not liking it. I would imagine it would work the same with cow's milk, though I guess depending on the reason your LO was on soy formula,you'll want to watch for any reaction early on in the transition
Re: If your baby was on soy formula