Nah, as long as you have configured permissions on the 2003 Exchange server you'll be fine. If something does happen you can always create a new account and switch the BES.
Here's a KB for the permissions needed for the account: BlackBerry Search Results
When the account is moved just update the MAPI profiles to point to the new Exchange Server. Follow these steps:
1. Move the mailbox and configure permissions
2. Open the registry
3. Go to HKEY_Current_Users\Software\Microsoft\WindowsNT\Cu rrentVersion\Windows Messaging Subsystem\Profiles\
4. Backup and delete the BlackBerry Server and BlackBerry Manager(BES 4.1 and up) profiles
5. Open the BlackBerry Server Configuration utility
6. Click the BlackBerry Server tab
7. Click Edit MAPI profile
8. Enter the Exchange Server name and mailbox then click Check Name
9. Reopen the BlackBerry Manager and click Ok to create the profile again (BES 4.1 and up only)
This should get up you upand running on the Exchange 2003 server.