Unfortunately you can't. Verizon uses and EVDO signal whereas Sprint, AT&T, T-Mobile, etc... all use CDMA signal. If you want an AT&T phone on Sprint you would have to buy a SIM card. But...
There are a wide range of locations that will recycle Verizon phones. Some of the more common places to recycle the phone are local retail stores such as Best Buy, Verizon Wireless stores, and...
No. You can buy an unlocked phone that is the same model and use it for another network but typically a Sprint phone will only work with Sprint and vise versa.