Marc,
To upgrade apply an Enterprise license to your Splunk Light instance and restart. There is no need to install Splunk Enterprise on top of your Light instance. Splunk will then start up as Splunk Enterprise with all the data inputs, report, alerts and other knowledge objects you have created. You will not need to rebuild anything. Keep in mind that once you move to Splunk Enterprise there is no easy way to go back.
The trial license is bundled with the Splunk Enterprise trial and not available as a stand alone license (as far as I know). I suspect that our sales team can get you a trial license but keep in mind that once you apply that license you will be running Splunk Enterprise and have a 30 day trial Once the trial expires you would revert to the free version of Splunk Enterprise with the limits that implies.
My recommendation is to work with our sales team and get a full Enterprise license or if you have a time gap of 30 days or less ask them to provide you a trial license. What you don't want is to move to a trial of Enterprise and then run out of time before you can get a full license as you would be stuck with no easy way to revert back to your Light license.
