Yea, the unlocked lock shows that the bootloader of your phone has been unlocked. This voids your warranty with at&T. If that bothers you you can remove it by using a paid root app called Triangle Away
Yes, but I'm not sure if the phone will be able to download the update over the air (OTA) since its original an ATT device. With that said, you'll be able to side load it so you'll be fine.