It is usually a straight forward upgrade. Ensure that permissions are set on the Exchange 2007 machine, uninstall ESM 2000 and install the Exchange MAPI/CDO 1.2 client from Microsoft. Check that MAPI profiles are seeing the new Exchange 2007 after installing the updated MAPI/CDO component.
That's pretty much it from the BES side. I do know that there are issues when you do not have Public Fodlers in Exchange 2007. It causes Calendar issues, here is RIM's KB with a more detailed explaination and fixes for it: BlackBerry Search Results
Other than that it's like going from Exchange 2000 to 2003. Make sure that MAPI/CDO are updated and permissions are set.
Hope that helps.