I had this same problem myself this weekend, and after 3 hours on the phone with Microsofts support, they told me there is nothing that can be done aside from making a new gamertag and purchasing the content again as this download binds itself to the system it was bought on originally rather than the gamertag.
OK, Apparently Microsoft silently released a license tool that fixes this problem.http://www.xbox.com/en-US/support/systemuse/xbox360/licensemigration/
You don't need the old 360, just the new one and Xbox Live, when you finish the transfer, redownload SW2XL and then launch from the SW2 disc - works fantastic.
I just wish they had actually ANNOUNCED this. Or maybe they did and I missed the memo. In any event I'm back up and running!