I read that ATT has already started updates too Windows Phone 8.1 for 1520 users. I have a international 1520 and have not yet seen the update. Does anyone know when this will be released for this version?
AT&T does not release firmware for non-AT&T distributed phones.
The update for your phone should come direct from Nokia once they have updated their firmware bits and released it.
The other side of this is if you have an at&t distributed phone, even after unlocking the phone and move to another carrier entirely, it will still look at at&t systems for updates. Unlocking it does not remove the at&t branding or switch you over to OE-direct update methods.