how to change a physics body shape on contact/collision - swift

Question

I have created a sprite with circle shape physics body. I want to change the circle shape into a rectangle shaped physics body on contact/collision with another body. I believe this should be done in the didBeginContact. Here's what I've done so far
ball.position = CGPointMake(self.frame.size.width/2, self.frame.size.height/2)
self.addChild(ball)
ball.physicsBody = SKPhysicsBody(circleOfRadius: ball.frame.size.width/2)
ball.physicsBody?.friction = 0
ball.physicsBody?.restitution = 1.2
ball.physicsBody?.linearDamping = 0
ball.physicsBody?.angularDamping = 0
ball.physicsBody?.allowsRotation = false
ball.physicsBody?.applyImpulse(CGVectorMake(2, -4))
func didBeginContact(contact: SKPhysicsContact) {
var firstBody : SKPhysicsBody
var secondBody : SKPhysicsBody
if contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask {
firstBody = contact.bodyA
secondBody = contact.bodyB
}
else {
firstBody = contact.bodyB
secondBody = contact.bodyA
}
if firstBody.categoryBitMask == ballCategory && secondBody.categoryBitMask == enemyCategory {
//change ball physics shape here
}
}

Answer 1

Remember that a physics body is something that belongs to a node. It's simply a property, and its dependent on that node to exist. All you have to do is swap one body for another in your node's physicsBody property:
//inside of didBeginContact, say you want to change firstBody's body to rectangle
firstBody.node.physicsBody = SKPhysicsBody(rectangleOfSize:...)