Decrypt and Extract your game using my tutorial here. Open your DecryptedExHeader.bin in HxD, and do a hexadecimal search with the UniqueID, reversed. So if your UniqueID is 17 BA, search for BA 17 (It's Bravely Second USA). Replace it anywhere it appear with the one you want. Do the exact same thing with all HeaderNCCH.bin files. It does not work with Pokémon Games [XY|ORAS|SM] due to the plainrgn.bin file. Very useful without Makerom, RSF, and other things. Thanks @xXDungeon_CrawlerXx for the help!