If I buy a Verizon phone, Will I be able to use it for AT&T?

I'm talking about buying a used phone on ebay (Lumia Icon or Lumia 929) and using it on AT&T. These phones were Verizon exclusives, but I have seen unlocked versions. I've also read that they can run on both GSM and CDSM networks. If I buy a locked version, will I be able to unlock it and use it on AT&T?

Read more: If I buy a Verizon phone, Will I be able to use it for AT&T?