We are currently unity 4.0(4)sr1 with Exchange 2000.
We will be moving to exchange 2003 .
To make the migration smoother is it possible to have both Exchange 2000 and Exchange 2003 message stores on the same Unity server?
So I can move the mailboxes a few at a time from Exchange 2000 to Exchange 2003?
Then remove the Exchange 2000 message store when all of the mail boxes have been moved.
Here is some additional infomation
To clarify: are you saying you will have a new Exchange 2003 server talking with your old Exchange 2000 server? Once you have the new Exchange 2003 server in place and it is communicating with you old Exchange 2000 server, you will need to upgrade ESM 2000 on the Unity server to 2003 based on this procedure: http://www.cisco.com/en/US/docs/voice_ip_comm/unity/42/upgrade/guide/ex/ru_110.html#wp1359133
And once that is done you will need to re-partner with the Exchange 2003 server http://www.cisco.com/en/US/docs/voice_ip_comm/unity/42/upgrade/guide/ex/ru_060.pdf
One note: I believe the Unity_Server account needs to be moved to the new mailbox store in order for Unity to work.
Once this is done Unity will be able to communicate with the old Exchange 2000 server where the mailboxes are via the Exchange 2003 partner server. Now you can move accounts.
I would move the accounts from Exchange 2000 to 2003 during a maintenance window though and test.