Few steps for migrating 2003 to 2008

After completing the forestprep check the schema version and wait for the completeing the replication and then run the domainprep.

How to check the schema version.
http://support.microsoft.com/kb/556086
Schema version for new operation system
Find the Schema version for the operating systems
13 - Windows 2000 Server30 - Windows Server 2003 RTM, Windows 2003 With Service Pack 1, Windows 2003 With Service Pack 231 -Windows Server 2003 R244 - Windows Server 2008 RTM47 - Windows Server 2008 R2
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Take the system state backup of your schema master then disable the outbound replication and run the forestprep.
Repadmin /options +DISABLE_OUTBOUND_REPL
After getting the success log enable the out bound replication via repadmin.
Repadmin /options -DISABLE_OUTBOUND_REPL
Wait for competing the replication.
Pls see the link also.
http://social.technet.microsoft.com/wiki/contents/articles/migrating-active-directory-domain-controller-from-windows-server-2003-to-windows-server-2008.aspx