Sorry if this has been covered already, I couldn't find a direct answer.
We're currently running Unity 4.2 attached to our Exchange 2003 server. Now we're getting ready to upgrade to Exchange 2007 so we figure that we need to upgrade Unity first. So we're planning an upgrade to Unity 5.0. I've built a couple of installs in our test system and I see that you can select to connect to an Exchange 2003 OR 2007 mailstore, not both.
Now for our Exchange migration, we're planning to build up a 2007 server and then slowly migrate users over (we have over 1000 at this facility). In my testing, I can see that it's quite simple to switch from Exchange 2003 to 2007, but what will happen during the migration period? If I click it over to 2007, does that mean the users on 2003 won't have Unified messaging available anymore? And if I leave it on 2003 until everyone is migrated over, is that going to mess anything up? I've read that if you have both servers, you need to be sure that Unity creates it's store on the 2007 server. So can I even do that after the install has already been completed with a 2003 server in place? Would it require another upgrade of Unity after 2007 is in place?