If you want to make a cut scene I'd say changing the graphic in the move event would be the easiest.
If you want interaction where new graphic = new functionality of the transformed NPC you should generally stick with making new pages. (You know, if his hair is black he'll say "Hi" and if his hair is white he'll say "Hello")
You don't need to use switches for that, but it's more stable to use and maintain than a no switch-using way. (There may be exceptions)
Let's have an example:
The local merchant got ill, so his youngest son has taken his place as the merchant.
This happened after the player have seen and talked with the local merchant.
The player comes back to the shop.
>Here we would like to another merchant, who sell other goods than before<
A way to utilize the