Well with DS1, he was such a picky eater and was doing so well on his formula. He was on Nutramigen though, hypoallergenic formula. When we did the gradual switch, things just got messed up.
Now with DS2, we tried to do the switch and it did not go well, then tried to put him back on formula and he was not drinking much, so went to Rice Milk.
I just say I wished we had stayed with formula, because you know they are getting the vitamins and nutrients they did.
In your case though, he is eating well and has been on a milk based formula, so it could be fine when you do make the switch.
What have you decieded to do? Are you going to try a lactose free formula, soy formula, soy milk, lactose free milk? When do you leave for vacation?