2D Shooter Make Item Pickup Change Bullet Prefab On Collision

I have a bullet script where the rigidbody(rb) is a prefab of one type of bullet, say earth and upon collision of another bullet type say fire, the prefab changes to fire. I have setup tags for each bullet but

if (collision.gameObject.tag == "RedBullet")
{
	PlayerShoot.Prefab "Then Item changes the Bullet"  == "RedBullet";
}

how do you tell the array to make the bullet active?