You must give in your phones as well to get Sprint phones.
Sprint will even give you up to $350 per phone line to cover early termination fees on your existing plan.
Please keep in mind that the discount is only for service charges. If you have a monthly fee associated with your phone, that will not be halved. Sprint acknowledges that most people will save closer to 20% with this promotion.
Ask your representative how much a Sprint plan will cost without this promotion, you may be surprised to find that it might actually be a better deal for you to ignore this promotion and just switch over on their regular rates.