Once completed, data is deleted from the Wii
The process of getting your data over to the Wii U from your Wii is a bit complex. The process does require your old Wii console, and you have to download the Wii to Wii U transfer application. You will also need an SD card with at least 512MB free, but we recommend more.
The process requires that you insert the card into the Wii U first and select the system transfer option, and then the card is moved to the Wii where you have to then download the transfer application from the Wii Shop. The Application moves the data to the SD card; and then once completed, you place the card into the Wii U and the data is moved to the new Wii U console.
For it to work, both the Wii and Wii U need to be connected to the Internet, and that is so you can download the application the verification can take place. The data that can be moved includes WiiWare, Virtual Console titles, save data for games, DLC content, Wii Points, and, of course, your Miis.
Once the transfer is completed, all of the data is wiped on the Wii console and the SD card is erased, so you have to hope you get it right. Seems like a lot of hassle, but if you are going to move to the new console, this is the only way that you are going to get there if you want to take your data with you.