Switching formula

Going to switch from Enfamil Enspire Gentlease to Enfamil Gentlease, because I recently started getting getting WIC and enspire isn't approved. Do you think it is better to just start my son on the new formula once the old is gone or should I give him a bottle of the new a day to transition him to it until the old is gone? What do you guys think? Just wanting to make the switch be as smooth as possible for him.