It's pretty simple, if you have bootable flash just navigate to folder "sources" open file cversion.ini and change "MinClient" value to 7000.
You can also do this with ISO image before burning it to dvd.
Just tested - works with Server 2008 R2 RC to Server 2008 R2 RTM.
One more thing, upgrade from windows, don't boot from usb or dvd it will ask you to boot up you windows and launch setup then.
