SQL Server Developer Edition Downgrading to Older Developer Edition
Microsoft gives Downgrade Rights to Volume Licensing customers and, to some extent, even to OEM and retail customers. 

However, in general, if you don't have a Volume License, you can't have the bits and the product key, which are necessary to excercise your downgrade rights.

Long story short, if you want 2008 R2, you'll have to buy it. 

Here's a blog entry of the licensing team on this subject