@Redinhell: I checked the terms and conditions of the Upgrade Programme, and its states that you should include the box and any accessories where possible when you return your old device. So, if you have the charger to hand, you should return it.
Hmm, interesting question. I personally do not know. I have never given that a thought as I have obviously never used that upgrade option. If I was to send a device back as part of an upgrade process, I will expect to send everything including the charger. I however, do not think it will cause any significant issues if you sent the phone and nothing else.