Just watch for reactions. Give a full dose of regular milk (milk tastes better than formula so kids usually go for it right away). Do that for a few days.
Be sure to do this when the kids are healthy and not sick. You want to watch for excessive gas, mucus or constipation. If you see any of these signs your child may not be able to digest the milk well. If thats the case try rice or soy milk, its still cheaper than formula.
My oldest was lactose intolerant. He was on Soy formula until he was three. We didn't make it full strength like for an infant. When he was three we tried milk again and slowly mixed it in with the formula. By then though he didn't drink as much maybe 1 or 2 glasses a day or just in cereal so it wasn't a big deal. He still isn't a big dairy fan and doesn't eat much cheese, yougurt or milk. I think thats ok as long as you are sure to give vitamins. I found these great kid vitamins that are gummy bears and the kids love taking them every day.
I also suggest taking the bottle completely away by 18 months at the latest. My youngest had some issues with the bottle and we finally just packed them all up and gave them away. We got one of those non-spill sippy cups that you sorta have to suck on to get it to let the contents out and he took that to bed, during the day he got a regular sippy cup. After a few months all the special sippy cups got "broken" and were regular cups too.