There may be a clever way to change it "in place" but I don't know of it - several things get tagged off the alias during mailbox creation, I think a lot of strings are attached to that one thread.
You could do a COBRAS backup (include message for just that one user) and then import them but change their alias (and display name or extension if you like). A bit of a long way around but it would do what you want I think.
"You modify settings for an individual user account from the pages available on the Edit menu in Cisco Unity Connection Administration.
Note that if you change the alias for a user, Connection automatically creates an SMTP proxy address for the previous alias. This allows other Connection users to reply to messages that were sent from the previous alias and have the replies reach the user at the new alias. When Connection is integrated with an LDAP directory, the Alias field in Connection cannot be changed for any user who is integrated with an LDAP user. However, if you are using Active Directory as the LDAP directory, you can change the value of the LDAP field that is mapped to the Alias field, and the change is replicated to Connection the next time the Connection database is synchronized with the LDAP directory. This also causes Connection to automatically create an SMTP proxy address for the previous alias."
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