Supported Upgrade Path for SQL 2012 can be found here:
Upgrade from SQL Server 2005 Evaluation, SQL Server 2008 Evaluation, and SQL Server 2008 R2 Evaluation to SQL Server 2012 is not supported.
So if you are just making a 2012 eval edition to a full enterprise edition it'll work. You can do so by starting setup.exe to bring up the installation menu.
Go to maintenance, then 'edition upgrade'
See the pic:
Or you could run the command line to install SQL Server but pass in the Product ID, using your own path, /q command optional.
setup.exe /q /ACTION=editionupgrade /InstanceName=MSSQLSERVER /PID=<appropriatePid> /SkipRules= Engine_SqlEngineHealthCheck