Moved over some blackberry users to 2010, along with the BESAdmin account and this still allowed blackberry users to work (all be it a bit slower) - even though they were still on 4.1.6.
4.1.6 does not support Exchange 2010, so you are on your own there.
What you should have done is:
- Setup BES 5
- Create and activate a test user (Exchange 2003) on BES 5.0
- Check if he activates and syncs ok.
- If that all works, use transporter to move everybody over from 4.1.6 to 5.0 (everyboday still on Exchange 2003)
- Once everybody is happy and working, move users to Exchange 2010 (there are docs how to do that).