– DLC files were encrypted with a console-specific key, meaning they could only be used on the 3DS that downloaded them.
: If your console has CFW, you can use GodMode9 to "dump" your installed DLC into a .cia file. This creates a permanent archive that can be reinstalled even if the eShop goes offline completely. Installing Archived DLC (.cia files) 3ds dlc archive
If you previously purchased DLC, you can still access your personal "archive" through the system settings: – DLC files were encrypted with a console-specific
format), you need a modded 3DS with custom firmware like Luma3DS. FBI Homebrew FBI application to browse your SD card and install the files directly to your Home Menu. Luma3DS LayeredFS 3ds dlc archive