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] http://msdn.microsoft.com/en-us/library/ms144259.aspx#Upgrade

[2] http://msdn.microsoft.com/en-us/library/ms143393.aspx

[3] http://msdn.microsoft.com/en-us/library/cc707783.aspx