Recently we discovered that some of the customers SQL 2012 Enterprise Edition Installations only uses 20 Cores (Machine has 64 Cores). After some Research we discovered, that there was a SQL 2012 Enterpriese Edition CAL License (PID/SKU) used. So to upgrade you can use SQL Server Setup to Change the product key to the SQL Core license key. If you are using Availability Group Function there might be warnings, but after upgrade our Environment with AVG works as before and now uses the 64 Cores.
More Informations could be found on the Blog: http://blogs.msdn.com/b/saponsqlserver/archive/2012/06/15/sql-server-2012-enterprise-editions.aspx
Supported Upgrade Szenario's you'll find on: http://msdn.microsoft.com/en-us/library/ms143393.aspx
Thanks for the post, this solved my mystery.