Change the edition in SQL Server 2008
Doing a SKUUPGRADE has changed a little bit in SQL Server 2008.
Instead of using the SKKUUPGRADE from the SETUP.EXE as in SQL Server 2005 you can now specify the parameter /ACTION=Upgrade. [1]
For the GUI friends, there is a new Edition Upgrade wizard which will help you to change your edition type (also useful if you first installed SQL Server Evaluation Edition and want to change to a non-Eval edition later) [3]
If and what editions you can upgrade from to is mentioned here [2].
[1] https://msdn.microsoft.com/en-us/library/ms144259.aspx#Upgrade