Migration of Work Items while upgrading the VSTS

There won't be a migration tool, so the best way is to export everything to Excel and then import it into your new install. This will lose all the historical data, but I guess that's the price we pay for using Beta 2.

Based on my recollection, you will lose descriptions, attachments, links and history. I don't recall what datatype the descriptions are, but apparently there is some issue in bringing those over via Excel. As for changing the destination, that's a good question. I don't think that there is (at least not in the current form). You might have to create a new spreadsheet connected to the new TFS and just copy/paste from the other spreadsheet. If you have both a Beta 2 install and a July CTP install, you could proof this out in that environment.