My contact in MS told me they don't support such an operation and many files must be modified with reg keys to achieve this. As I told above the maintenance operation done when activating must be reverse engineered. You will have to download the trial version and create a different repository for that.
But if you want to go the hard way, let me give you an option. Install the trial version and activate your license key by running maintenance mode while monitoring with these tools and see what changes are made. I recommend doing this on a virtual machine or a test machine that you never setup trial or full version. This way you can see what changed while activating, and you can reverse the operation if it's possible to do.
http://www.pcmag.com/article2/0,2817,25198,00.asphttp://www.epsilonsquared.com/http://www.ghacks.net/2009/03/01/software-installation-monitor/http://www.innovative-sol.com/uninstaller/manual/installation_monitor.htm
