I have heard that it is possible through Nintendo Customer Support to download software that is not currently available on the DSi Shop (but once was) if you can prove you purchased/downloaded it when it was available. From what I understand, if you call them and give them the details of your issue, for instance if you bought software on one DSi and it's broken, and would like to transfer it to a new DSi, you can give them the serial number from your first DSi to prove that you bought it, (they can check a database), then give them the serial number for your new DSi, and they can give you a special code to download the software onto your new DSi. I am curious to know though, where would you even enter this code at once they give it to you? I've looked around the DSi Shop menus and can only find a spot to enter the code to redeem DSiWare Points...not a code for a specific piece of software.
I have a DSi XL with Zelda: Four Swords Anniversary Edition and it has a broken hinge and cracked screen, and is just basically falling apart, so I bought a new DSi XL and would like to transfer Four Swords Anniversy Edition to it. Of course transferring is not possible, and I know it's a long shot, but apparently, according to DELTATROPE here: http://techforums.nintendo.com/message/147654 it is possible to download it onto the new one, even though the game is no longer readily available on the DSi Shop. You just have to prove you got it when it was available. I'd just like to know exactly how this 'special code' works before calling, so I can better explain everything to the customer service rep.