You could follow previous recommendation and also recognize the toppings and anything else that you want to change.
Your input> I want to change to large
# Detecting the intent_change and entity `large` size found using custom action to change the slot to large
> Changing to a large pizza
Your input> I wan to add mushrooms
# Detecting intent_change, entity `add` (modifier?) and entity `mushroom` topping
> Adding mushrooms to your pizza. Your pizza has the following toppings: pepperoni and mushroom
This is something you would have to program in the custom action to deal with the entities found in the user input.
Additionally, you could create a new form that asks what you want to change (size, toppings, etc) and let you re-answer the question. You could use dynamic form behavior to decide what slots to request again based on your answers.
Your input> I want to change something in my pizza
> What would you like to change: size/toppings/crust
Your input> size
> What size would you like?
Your input> large
> Ok. Anything else? size/topping/crust/done