If this is the first time you are reading this and you´ve already traded-in your Wii this may piss you off. However, the fact still remains that according to gimmie gimmie games who reached out to Nintendo themselves. Yes, you will need your original Wii in order to transfer Wii Ware, downloaded games, and any other content onto the Wii U.
Here is the exact email response from Nintendo of America:
Thank you for writing. I can confirm that you will be able to transfer your content from your Wii to the new Wii U. Wii U owners can wirelessly transfer content such as Miis, and WiiWare and Virtual Console games to their Wii U systems. Both systems will need to be connected to the Internet and the Wii U will need an SD card to facilitate the transfer. The process is very similar to transferring content from Nintendo DS to Nintendo 3DS or from Nintendo 3DS to Nintendo 3DS XL; which has been received positively by consumers. Transferred Wii content is accessible and can be played through Wii Mode, an application on the Wii U’s main menu.
Nintendo of America Inc.
So, if you’ve already traded in your Wii system with all of your WiiWare and Virtual Console games to GameStop then you’re screwed. You won’t be able to get those games back on your Wii U, you’ll need to repurchase them. I actually feel bad for Gamestop cause they are going to give out a ton of store credit and get back a bunch of useless Wii´s all at the same time.