How can I change character?

I don't know how can I change character, I have two characters a ball, and one avatar, my avatar can transform in that ball, I have those models and animations, but I don't know how can I change it, I tried with a Empy Game Object and a simple script, like this:

 public GameObject Avatar;

 public GameObject Ball;

 public KeyCode inventoryKey;
 public KeyCode inventoryKey1;


 // Use this for initialization
 void Start () {

     Ball.gameObject.SetActive(true);

     Avatar.gameObject.SetActive(false);
     
 }
 
 // Update is called once per frame
 void Update () {

     if (Input.GetKeyDown(inventoryKey))
     {

         Ball.gameObject.SetActive(true);
         Avatar.gameObject.SetActive(false);

     }
     else if (Input.GetKeyDown(inventoryKey1))
     {
         Ball.gameObject.SetActive(false);
         Avatar.gameObject.SetActive(true);
     }
     
 }

}

But it daesn't work, and I want to change those if for collider, like this:

OnCollisionEnter(){ Active my avatar and desactive the ball } and when the collision finish active ball and desactive avatar.