Milk Transition

Hi moms. My baby is turning 1 next month. Paano ba magswitch ng milk. Bonnamil kami now, going Bonakid. Sa mismong bday ba niya? Or pwede mas early? Need ko ba imix or what para hindi mabigla tyan niya and magka-upset stomach? Please advise. FTM here. Thankyou in advance. 💕