Important Information:
Both the Wii and Wii U must have the most recent menu version installed. How to
If both systems are not connected to a television, you will need to switch the television connection between systems in order to complete a system transfer.
You can either:
Use the input select of the TV to switch between the two systems
Plug the standard AV cable that came with your Wii console into a TV and swap the cable between the two systems during the transfer process. (Swap the cables at the same time you are swapping the SD card.)
Transferred data is permanently deleted from the Wii console. Once you have transferred data to the Wii U console, it cannot be transferred back to a Wii console.
Data cannot be transferred from your Wii U console to a Wii console.
To view a list of what you can and cannot transfer click here.