Note. When Exchange is installed on this particular server you can use this procedure only in a lab environment. To change an Exchange server is not a supported scenario!
When installing an Exchange 2010 environment in my lab I discovered that the Fail Over Clustering bits were not available on my planned DAG members. It turned out that I installed Windows 2008 R2 Standard Edition instead of Enterprise Edition. Even worse, Exchange Server 2010 SP2 was already installed as well.
On TechNet there’s an article that explains how to Upgrade Windows 2008 R2 without using the installation media (i.e. reinstall Windows 2008 R2 from scratch) using DISM, the Deployment Image Servicing and Management Tool.