Please click on this link and find your Iphone model this might help you alot
https://www.whistleout.com/CellPhones/Guides/Taking-your-ATT-iPhone-to-another-carrier#heading-7
Thanks
r0ot
its depends on your model also as i know so far there is two model of Iphone 7 compatible to change the carrier
iPhone 7 & iPhone 7 Plus from AT&T (Model #A1778) and (Model #A1784)
Thanks.