If the SD card is above 32GB, it must be formatted first: Formatting an SD card|Don't use the SD card in the 3DS prior to copying data to it, or games and other applications may not appear.|Copy all the files over to the new card.
If the SD card is above 32GB, it must be formatted first: Formatting an SD card|Don't use the SD card in the 3DS prior to copying data to it, or games and other applications may not appear.|Copy all the files over to the new card.