I’ve been asked this many times in the past two days. The WiiU supports transfering all the data from your Wii to itself (with cute Pikmin carrying all the data). The problem is that all the data is deleted from the Wii, nearly restoring it to a factory state. This can be quite undesirable especially for those who intend to keep using their Wii for other reasons. This requires a Wii already running homebrew.
To solve this is pretty simple:
- Make your Wii homebrew ready (https://sites.google.com/site/completesg/about/system-menu-4_3)
- Install Homebrew channel and BootMii onto your Wii (http://bootmii.org/download/)
- Reboot your Wii to BootMii and select the Gears Icon and then the Nand Dump Icon (chip -> sd card)
- Now perform the entire Wii -> WiiU Transfer process (long process)
- Reboot your Wii to BootMii and select the Gears Icon and then the Nand Restore Icon (sd card -> chip)
Everything except any virtual console games installed to your SD Card will come back.