How to get kitty used to a variety of food?

I want to get my new 3.5 kitten used to a variety of food/textures in case she has to eat a prescription diet in the future, but I don't want to upset her stomach in the process. How should I approach this? Currently she's eating Purina One Kitten (she's free fed this) and Fancy feast ocean whitefish and tuna (given to her 2x a day).