My ped told me to just do a switch all at once, BUT i would not do that. I would take it slow like the others said. If she is doing a 6 oz. bottle, I would do maybe 2 oz. gentlease and then 4 oz. Lactose Free for a day and then 3 oz./3 oz. the next day and then 4 oz. and 2oz. the next and so on.
The Gentlease only has 1/4 of the normal lactose than regular formula, so you know if she can't handle that little bit, she want be able to handle any at all.
Oh and just FYI, the Gentlease gave my lo poos that smelled like rotten eggs. May just have been him because he cannot tolerate milk at all, but I asked my friend about it and her lo's ones poos smelled really bad with it.
Good Luck