For once, you are going the other way with this. Most people are moving from Voicemail only to Unified Messaging. The concept you need to take with this is the following:
Being they subscribers are currently in UM, Im not sure what is the best way to address the voicemails from old system to new system. It might be easier for them to start "fresh" on the new system with no new old voicemails being migrated over. They can still listed to old messages as wav files in their current email platform.
Run a DIRT backup of the system. This will grab all SQL database, configuration settings, subscribers, etc.
Re-install the Unity server in it's own Voicemail only domain. You will of course need to setup up this domain with AD, DNS, Exchange, etc. Once setup, install Unity as you were installing a new system from scratch.
Make sure you match the Unity versions of the old Unity to the new-reinstalled version.
Integrate back to CUCM
Test Unity
Use DIRT to restore your prior Unity setup. Unity DIRT will see these accounts have not been setup up yet and create new AD and Exchange accounts for you.
All user greetings, personal greetings, etc will come over from the old system.
make sense?
cheers!
