at 18 months she should be ready to switch over entirely. Soy would not be appropriate for a child unless she has milk allergies. Feed her whole milk, it's incredibly important for her brain development.
No, no it's not.
http://www.slate.com/articles/health_and_science/human_evolu...
https://www.choosemyplate.gov/dairy-nutrients-health
http://www.idfa.org/news-views/media-kits/milk/importance-of...
Bovine and goat milks are unsuitable for infants under 12 months, because they contain too much protein in undiluted form, and not enough nutrition content when diluted to safe protein levels.
The dawn of civilization up until the industrial age fed human babies exclusively on their mothers' milk, or upon the milk of a human wet nurse. Those that could not feed in this manner died. It was very tragic, and it made a lot of people very sad.
Consuming bovine milk is not a critical step in human evolution. It's just a culinary choice. People also ate locusts back then. It didn't catch on as well, because cheese tastes better, and doesn't have raspy, sticklike hindlegs that get stuck in your teeth if you don't snip them off with kitchen scissors. You can certainly feed your baby pureed grasshoppers, but most people don't, because they are squeamish about insectophagy themselves.
https://www.sciencedaily.com/releases/2009/07/090722083720.h...