How to change physics body and sprite image of existing sprite after contact - swift

Question

I have created two sprites with physics bodies and handled their contact. How can I change the physics body and sprite image for one of sprites when the contact occurs? Please see relevant code below:
// circle Sprite
circleSprite.name = circleSpriteCategoryName
circleSprite.position = CGPointMake(2*self.frame.size.width/3, 2*self.frame.size.height/3)
circleSprite.zPosition = 10
self.addChild(circleSprite)
circleSprite.physicsBody = SKPhysicsBody(circleOfRadius: circleSprite.frame.size.width/2)
circleSprite.physicsBody?.friction = 0
circleSprite.physicsBody?.restitution = 1
circleSprite.physicsBody?.linearDamping = 0
circleSprite.physicsBody?.angularDamping = 0
circleSprite.physicsBody?.allowsRotation = false
circleSprite.physicsBody?.applyImpulse(CGVectorMake(4, -4))
// square sprite
let squareSprite = SKSpriteNode(imageNamed: "square.png")
squareSprite.position = CGPointMake(CGRectGetWidth(frame)*0.4, CGRectGetHeight(frame)*0.8)
squareSprite.physicsBody = SKPhysicsBody(rectangleOfSize: squareSprite.frame.size)
squareSprite.physicsBody?.friction = 0.0
squareSprite.name = squareSpriteCategoryName
squareSprite.physicsBody?.categoryBitMask = squareSpriteCategory
squareSprite.zPosition = 10
addChild(squareSprite)
// contact in didBeginContact
if firstBody.categoryBitMask == circleSpriteCategory && secondBody.categoryBitMask == squareSpriteCategory {
println("contact works")
}
EDIT 1
I would like to change update the physics body and image of circleSprite to have the following properties:
circleSprite = SKSpriteNode(imageNamed: "recto.png")
circleSprite.physicsBody = SKPhysicsBody(rectangleOfSize: circleSprite.frame.size)
circleSprite.physicsBody?.friction = 0.0
circleSprite.name = newSpriteCategoryName
circleSprite.physicsBody?.categoryBitMask = newSpriteCategory
EDIT 2
I managed to get change the sprite image thanks to help from Max_Power89. But the image has been reduced to the dimensions of the physics body. Please see the relevant code below:
if firstBody.categoryBitMask == circleSpriteCategory && secondBody.categoryBitMask == squareSpriteCategory {
if let newSprite = firstBody.node {
let newImage = SKTexture(imageNamed: "newSprite.png")
let action = SKAction.setTexture(newImage)
newSprite.runAction(action)
}
}

Answer 1

You can implement SKPhysicsContactDelegate
then in your didBeginContact delegate you can do something like this
func didBeginContact(contact: SKPhysicsContact) {
// Return whatever it's the other body
let other = contact.bodyA.categoryBitMask == PhisicsCategory.Player ? contact.bodyB : contact.bodyA
if other.categoryBitMask == PhisicsCategory.Playe1 {
//Change sprite
let deadTexture = SKTexture(imageNamed: "Your image name")
let action = SKAction.setTexture(deadTexture)
player1.runAction(action)
}
}