How can I move my old cell phone to a new carrier?

Episode 1188 (2:01:11)

Kris from Merced, CA
Apple iPhone 6

Kris would like to move her iPhone 4 from AT&T to Verizon. Leo says she can't. The phones are the same model, but the iPhone 4 on AT&T has a GSM radio, and the Verizon model uses CDMA. She could get it unlocked and use it on T-Mobile, though. But to go from AT&T to Verizon, she'll need an iPhone 5 or above. In fact, if she can go to Verizon and say she wants to leave AT&T. Verizon actually sells an unlocked iPhone due to a consent decree with the federal government. And then she could use it on any carrier.