Does a Windows Server 2012 R2 Standard license cover an installation of a Windows Server 2008 R2 Enterprise?
Yes, a Windows Server 2012 R2 license entitles you to a version downgrade as well as an edition downgrade. A Windows Server 2012 R2 standard license allows a downgrade to the previous versions of the editions Standard, Enterprise, and Essentials.
For a Datacenter license, you can downgrade to the previous versions of the editions Standard, Enterprise, and Datacenter.
Source: Microsoft Windows Server 2012 Licensing & Pricing FAQ as of 2017-03-19.pdf (page 8)