To which product can I migrate a Microsoft Project Server 2007 External Connector license (end-of-life)? What conditions must be met for the migration?

Microsoft Project Server 2007 External Connector licenses can be exchanged for Project Server 2010 Client Access licenses in a 1: 500 ratio if the External Connector licenses were under active Software Assurance at the release date of Project Server 2010.

Source: Microsoft Product List September 2012 (Page 135)