A frequently misunderstood area of Microsoft licensing is knowing what rules apply when you are utilizing downgrade rights (the right to install an earlier version of the product under a newer license).
The version purchased determines the use rights regardless of what version is installed.
However; this gets a little more confusing for companies who maintain Software Assurance on their products. For example if a customer bought a license for Microsoft SQL Server Enterprise in July of 2011 with Software Assurance (we’ll assume 3 full years of Software Assurance) they would have bought the rights to Microsoft SQL Server 2008R2 Enterprise (either per server or per processor) and enjoyed upgrade rights to later versions of that product. If they choose to run 2008R2 (or an older edition), then the 2008R2 rules would apply. If they choose to upgrade to 2012 then the 2012 rules would apply.