If you are talking about changing one Object:Sprite (Sprite 1) to another Object:Sprite( Sprite 2); you can code it a multiple of ways.
After the event where you 'touch' or 'collide' with it. Destroy Create Sprite 2 to the position and layer of Sprite 1, then destroy Sprite 1.
Alternatively If Sprite 2 is always positioned to Sprite 1. At the start of the layout turn Sprite 2 invisible. Then once Sprite 1 is touch, turn Sprite 1 invisible and Sprite 2 visible.
However, as has been suggested. An easy wasy is to have Sprite 1 form the first frame of an animation and Sprite 2 to form the second frame. At the start of the layout you stop the animation and set the frame to the fisrt. Then afte the 'touch' even occurs, you change the animation frame to the second one. Creating the illustion of the Sprites switching.