How to turn an Verizon phone (iPhone) into an At&t phone? - 1

What do I have to do?

Verizon and Sprint use a CDMA Network. They do not use SIM cards in their phones. The iphones you purchase from them have CDMA antennas to communicate with the network. ATT and T-Mobile use GSM which requires a SIM card. Apple also installs different software for all the carriers. Basically, you need to sell your iphone and buy a new one because the hardware is completely different and if you try to replace it the software won't match and it will ruin your phone.