First, make sure you have bootmii in boot2. Stop if you don't have it because dangerous to do anything on a skined system menu.
Make a NAND copy. Then use Modmii to download the following,
* System Menu 4.1U
* IOS 80
* IOS 62
* System Menu 4.3U
Say no to all patching. Then install them using a WAD manager in exact order shown above or below,
* System Menu 4.1U - First
This will remove the Mario Skin
* IOS 80 and IOS 62 - Second and third
Needed IOS for System Menu 4.3U and Wii U transfer
* System Menu 4.3U - Last
Once you done all of that go to Wii Shop Channel and download Wii U transfer channel. You can only transfer legitimate stuff to Wii U (no backup, pirated stuff).