You can’t downgrade any Microsoft OS to an older one from within the OS. You can however use your downgrade rights to install an older copy of the OS on that machine. This allows you to purchase Windows 2008 R2 and instead of using Windows 2008 R2 you can install Window 2008, or Windows 2003. If you are using an OEM Windows 2008 R2 license you’ll need to contact your OEM provider for media and a Windows 2008 Key.
There will be a small cost which goes with this, but it shouldn’t be more than $100 US.