Yes you can port a number. Please verify that you are a retail authorized user on the AT&T account.
Are you certain that Verizon meets your service needs? You don't have to port at purchase. You can get a new Verizon iPhone 5 test the service and port after you are happy with the service.
What to Bring with You
Your old mobile phone, if youre replacing it.
Your Social Security number.
Valid government-issued photo ID.
Carrier account password, if you have one.