On the unity side, when migrating from exchange 2003 to 2007, is there anything that needs to be done?
First of all make sure your Unity version supports Exchange 2007! It should be 5.X or later as you have 7.X you are good to go!
Then, as the partner exchange server will be changed you will need to run the Message Store Configuration Wizard again and check the permissions too.
Please refer to this link, it covers the procedure you need to follow:
*** Upgrading Exchange 2000 or 2003 to Exchange 2007 for Cisco Unity 5.x Without Failover ***
If this helps, please rate :)