I echo Mimi 2's comments. I had a lot of people (including my pediatrician) suggesting we make the change cold turkey b/c DD "would like the milk much more than formula since it is sweeter". She did not. As with all things, making changes slowly seems to be best (at least for us). Beginning at about 13 mo, I started adding 1-2 ounces whole milk in lieu of formula until she was at all milk. It took only a couple of weeks. The harder transition was from bottle to sippy cup, but still by 15 mo, DD was off the bottle completely. Like Mimi says, the concern is that baby is going to bed with the bottle and having milk pooling in their little mouth. BTW, I still warm up the milk at night (I think it helps DD with the whole bedtime routine), but not in the morning.