If a change of milk is indicated, from breast feeding to formula feeding for example, it is best to do it gradually. Make sure baby is healthy and not sick in any way, and do not rush the process.
Try not to change the milk during the first feed of the day when the tummy is not fully awake. Nor for the last feed before putting baby down to sleep - an upset tummy can make baby restless and keep everyone else awake too!
By the same token, it is also wise not to change all of the meals on the same day. Allow baby to get used to the milk formula gradually until baby becomes accustomed to it.
Consult your doctor or paediatrician as soon as possible if your child has trouble nursing, is not eating enough formula each day, or cannot take breast milk or formula for more than 1 day. And if baby has diarrhoea or is vomiting. This may indicate a change of formula is needed or something else should be checked.