Hi pika,
my boy on stage 2 milk 1 week ago. He on Nan Pro 2. He starts with Nan HA 1, then switch to Pro 1 and this switched takes longer as he was 3 month old that time but the recent switch from Pro 1 to 2, he takes 1 weeks to switch.
Start mix w 1 scoop stage 2 and the rest stage 1, then 2nd day increase stage 2 to 2 scoop, 3rd day 3 scoop, slowly increase till replace all and at the same time monitor baby poo.