Well, we've spent all day trying to do this at our house and we're stuck in limbo now. We did the migration, but now, we can't get DLC on either Wii console without re-downloading every song one at a time. The old Wii doesn't have the DLC and the Wii U refuses to download anything over a certain size because it seems to think we're limited to the base storage of the old one. We used an SD card in the old one. The SD card apparently can't be used to store stuff for the new one.
Anyway, has anyone gotten their DLC to transfer from the old Wii to the Wii U? If so, can you explain how you pulled it off? We've looked at the stuff both from within Rock Band itself and from the Wii store and can't figure anything else to try.
Thanks in advance for any help you can offer.