No, nothing says you have to, but you'll often lose out on the savings. You could try to sell it secondhand to recoup some of the costs of purchasing the new phone (especially if you buy unlocked), and you can sometimes get more cash than the trade-in deals the carriers or stores offer. That, however, requires more footwork and effort on your part and only takes away the sting of the new phone's cost in the long run. So it all depends on what you're looking for. If you're ready to say goodbye to your old phone, it's by far easier to trade it in or hand it down to a friend or family member.