I’ve been around in the Windows world for quite some time and have studied quite a bit as well. As such I have always been taught that upgrading the functional level was irreversible. It also didn’t help that I got my Windows 2008 certification through beta exams before R2 was released and even before there were any books available.
Thanks to this blog post however, it came to my attentention that it is possible to lower the Forest and Domain Functional Level from 2008R2 to 2008 or from 2012 to 2008R2.
All of this can only be done using PowerShell as you can read in “Step by Step guide to lower Active Directory functional level” by Aman Sahota.