Hi -
Yes - you can reset the password for this account in AD. Caveat if you do, make this change during a scheduled downtime as you will want to stop/restart Unity to test it. Meaning your services will continue to run using the existing account/password until you stop/start Unity or reboot. You will need to rerun the following Unity utilities - here is the order:
1. Change password in AD
2. Rerun the Services wizard - this will change all services that use the Unitymsgstoresvc account and is a lot easier than changing each service manually
3. Rerun the Message Store Services wizard
4. Stop/restart Unity or reboot
5. Test sending voice message and monitor Application/Event system logs
Here is a structured approach using the reconfiguration guide - http://www.cisco.com/en/US/customer/products/sw/voicesw/ps2237/products_upgrade_guides_chapter09186a0080205a83.html
Ginger