Thanks ladies! :)

I would add half of each formula. So if the baby is eating 4 oz one scoop of the sensitive and one scoop of the advance. Then gradually decrease the amount of sensitive until you no longer have to use it.

Gradually add the new formula to a bottle of the original, until there is more ams more new and less original, and finally just straight new formula. It will be easier on the tummy. You don't wanna overwhelm baby, and it might take a few days to get used to it.