Hopefully the responses in this forum posting will help.
If the link is no longer available:
One suggestion was:
1) Upgrading from trial Edition prior to the end of the period
To perform an in-place upgrade of SQL 2008 trial edition to other SQL Server 2008 full blown editions, simply run the setup and it will identify the existing installation, and will run an upgrade check. It will then let you know if the upgrade is possible or not. If the upgrade path is supported, simply go through the setup wizard and it will automatically upgrade the already installed instance of trial edition.
2) Upgrade from trial Edition 2008 after it has expired
You need to uninstall the trial version of SQL Server 2008 Edition before installing the fully licensed version of the other editions.
Second suggestion was:
Step 1: Run the SQL Server setup.exe again.
Step 2: In the SQL Server Installation Center, click "Maintenance" on the left navigation bar. On the right pane, click Edition Upgrade.
Step 3: Go through the wizard and you'll be able to enter the license key.
Verified by upgrading a SQL Server 2008 Instance from Enterprise Evaluation to Enterprise
Have to note that the first person's suggestion was by someone who was labeled as "Microsoft Contingent Staff".