I successfully debranded my AT&T Lumia 520. I used the Brazilian Country Variant Rom which amber is available for and now, it isn't branded to AT&T. Since the AT&T Lumia 520 has a firmware starting with 3043, I recommend using an amber rom to debrand it (3046.XXXX.XXXX.XXXX). You can use Nokia Care Suite to download the rom and then update it.
Thanks for the reply. Did you get any sharing problem after changing the ROM?