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 this is not possible on Verizon.
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.
yes if you have a iPhone with at&t . the way to switch iPhone carriers is take the iPhone sim card out and put a t moilble sim card or Verizon or sprint or boost moilble or whatever phone company