Transition to FM/UHT milk

Hello!! LO is 16mo now, full latch day and night on demand. Planning to start him in a childcare few months from now, so I'm trying to introduce FM and UHT milk in the day, but he rejects. Could anyone share their experience to transit LO to scheduled milk feedings and drinking from milk packets/cups? Any advice to prepare them for school or night weaning? Thanks a lot!! 😊