My intuition tells me it comes pre-configured if you are on official carrier.
Is LTE available wher you live.
I doubt it.
If so, AT&T would spend countless hours assisting all their customers to change the APN settings.
Besides that, the network panel is not even open when using an AT&T sim in an AT&T locked iPhone.
I am sure it will be preconfigured.
Yes it is. But I'm going to buy an unlocked iPhone 5 then use my current AT&T SIM card. I assume the APN changes for LTE.