[FLASH] How to use NCS to change from RM-820… | Nokia Lumia 920 | XDA Forums
Not really known 100% at this point. When I flashed my ATT 920 to Rogers pre-portico rom, it let me do the OTA upgrade to the rogers portico rom. Not sure if this will still work if you change an RM-820 into an RM-821 but it seems like it will....