Switching formulas

Darcy • 31 y/o happily married. Can't have a rainbow w/o a lil rain. 💦🌈 😇mom to 2 Angels &1 rainbow due 3/24/16. IUI/ Clomid
I am primarily breast feeding/pumping but supplementing with some formula and I was using Similac Advance but just received some samples of Enfamil Newborn so I'd like to try that- does anyone know if I can just switch formulas regularly or do I need to do it slowly so as not to upset her tummy?