I think that you can upgrade.
See http://www.adobe.com/products/photoshop-elements.html and click Upgrade in the right rail.
In the highlighted portion, choose the values for the software you own: version, platform, and language.
Make sure that you choose correct values. The new installer wil ask for the older serial number, and if these values are incorrect that the serial number will be flagged incorrect.
If you already have the trial version, you can enter the serial number you get after upgrading.
Europe, Middle East and Africa