Once you start moving the users over to the new Exchange server you will need to upgrade the BES.
Do the following in this order. Upgrade the BES to SP1 for Windows 2003. You will also need to apply SP4 HF3 for BES 4.0. Then you will need to apply SP2 for Exchange 2003 on the BES and reboot. This will update the mapi32.dll and cdo.dll on the BES.
After you have applied SP2 for Exchange, open the MAGT log file and search for cdo.dll and make sure the file version is 6.5.7638.x. (It should be at the bottom of the log file.)
Before you start moving users over to the new Exchange server you will need to make sure your service account (BESAdmin) will have the following permissions: http://www.blackberry.com/knowledgec...?nodeid=278513
Once this is done you can start moving the users over to the new Exchange server. After you move users you will need to run "handheldcleanup.exe -u" on the BES. This will look for any users who's mailboxes have been moved to a new Exchange server.
Hope this helps