Milk Transition

My LO is 15 months and is currently on Nan. We would like to transition him to fresh milk as we read that its better for his health and it saves cost too. We've read up a lot online but are skeptical on how to take the first step as we are afraid that our LO will have a negative reaction to it like stomachache or vomitting. My LO still wakes up at least 2-3 times at night for milk so if i have transitioned to fresh milk, can i still give formula milk at night only or how do i go about with that? Can any of you please share your experiences and recommend brands that we can use as well please.