During an Active Directory Upgrade project, I have collected many links and reference articles around upgrading Active Directory from Windows Server 2003 R2 to Windows Server 2008 R2.  Since these types of projects have now been around for awhile, there have been many lists compiled by Microsoft professionals collecting this information.  Therefore, this is only a collection of those findings and another reference to those that did the initial hard work for us all!  The first link is the most comprehensive collection from internal discussion throughout Microsoft internal DLs.

Keep in mind, that before you dive into these links and articles, support professionals should do their due diligence of checking event errors and logs first, as many of these will reference some of those known Event IDs.  Sometimes I would search the entire string of the error message to find related posts on other forums of Blogs on BING!

Another related Blog post from this project was Comparing Windows Server 2008 R2 Server and Core Roles and Features, as the customer wanted a side-by-side listing of what you might not gain if you only installed Server Core versus a full installation.