I just ran this to re-upgrade my developer edition back to standard:

:: You need disk 1 in servers and disk 2 in tools underneath c:\sqladmin\sql2005

@echo off

Set ADDLOCAL=SQL_Engine,SQL_Data_Files,SQL_Tools90,SQL_Replication

Set INSTANCENAME=MSSQLSERVER

Start /wait c:\sqlAdmin\SQL2005\servers\setup.exe /qb UPGRADE=%ADDLOCAL% SKUUPGRADE=1 INSTANCENAME=%INSTANCENAME%

And it went through ok