Aww, it sounds like your little one's tummy is sensitive sa formula transition, kaya nag-adjust lang talaga, yung watery poop ay common when switching to a new milk kaya okay lang na bumalik muna kayo sa 6-12 months para ma-normalize ang digestion niya. Since 1-3 ay para na sa older toddlers, baka talaga ang tummy niya ay hindi pa ready, pero better to have your pedia check kung okay lang ba itong gradual transition or kung may allergy concern, kasi they can guide you on the best timing at method para sa baby mo. Ang alam ko lang from my mum friends na nakaranas din nito, some babies need a slower switch (like mixing old at new milk gradually) instead of going cold turkey, pero talaga na case-to-case ito, better safe to confirm with your doctor po!