With my son I started when he turned 11 months. In his 8 oz bottle I would put 6 oz of formula, 2 oz milk for a week. The second week I did 4 oz formula, and 4 oz milk. Third week 2 oz formula and 6 oz milk. Then the last week, by the time he was a year it was all milk, never had an issue with him refusing it

I just started doing a quarter or a third cow's milk for a couple days and then half and then a little more than half. He's never had any problems with dairy so far.