SQL Server Trial Edition to Full Edition

What you want to do is treat it like an edition upgrade. Grab your VL copy of SQL Server, start the install, select the maintenance tab, and choose edition upgrade. See this blog posting on MSDN:

http://blogs.msdn.com/b/wesleyb/archive/2010/05/03/can-i-upgrade-my-sql-server-2008-r2-trial-to-a-full-version.aspx

I know this is an old post but since it comes up on top in a Google search I thought I'd add my findings for SQL 2012 and Open Volume License which probably also applies to 2008

From the x64 directory of the SQL 2012 Standard Open Volume installation media there is a file named DefaultSetup.ini

That file contains the PID installation key required to do a version upgrade/downgrade.