When upgrading to Exchange 2010 the first thing you must check is that your Exchange 2003 organisation is in Native mode. By default all Exchange 2003 installations are in mixed mode.
Exchange 2003 mixed mode allows 2003 to work with legacy Exchange 2000 servers.
Native mode does not allow Exchange 2000 servers and provides enhanced functionality. To understand the improvements in Exchange 2003 native mode please see the following article:
If you have any Exchange 2000 servers you must remove them before being able to upgrade to Native mode.
To upgrade Exchange 2003 to native mode open system manager, right click the organisation (mines Test Lab) and click properties.
Click Change Mode to change to Native Mode.
Now you must prepare the legacy Exchange 2003 permissions. This ensures the Exchange 2003 Recipient Update Service functions correctly after you update the Active Directory schema for Exchange Server 2010. For more information on this please see:
To do this run the following command of the Exchange 2010 media:
Note you will get an error if you haven't upgraded Exchange 2003 to native mode:
Lastly prepare the schema and domain for Exchange 2010. You can do this by running the following command:
Congratulations you can now migrate to Exchange 2010