Tried this but it completely f**ked up the user account, so the Wii U could no longer use it and forced to create a new user. This ofc. changed the user ID, so now the old saves don't work anymore...You can actually make the account act like it has no NNID and remove it directly from the user settings as normal, which should also remove most left overs from the account.
So to all people reading this: Take it with a grain of salt. It might work in most cases but sometimes there seem to be issues.