For any current user in Unity Connection that is also found in your LDAP directory, all the sync will do is update certain user fields of that user, it will leave everything else about their voicemail account alone. Their existing messages, recorded greetings will all be left intact.
You can see exactly which user fields these are by going into the 'LDAP Directory Configuration' and creating a new LDAP directory (you don't have to actually create one yet, clicking 'Add New' will just show you the settings). Do make sure that the user's extension number is entered correctly in your LDAP directory, that's the most important one to get right!
Any users in your Unity Connection setup that are not found in your LDAP directory will be left exactly as they are, although if you ever do create an account with the same username in your LDAP directory then these will be synced up during the next scheduled sync.
When you then turn on LDAP authentication, that's the point at which users logging into the Cisco PCA will start using their LDAP password to login. If you were ever to turn this off again it would revert to whichever manual password you'd set for that account.
Hope that clears it up, we did this for our own Unity Connection server a few months back for a few hundred users and it worked perfectly, but there is a lack of clear information about how all this works out there when it comes to LDAP. Doing the same on a CUCM server is a lot more terminal if you get it wrong as any account that is not also in your LDAP directory is deleted a couple of nights later, you don't get to have a mix like in UC!
All the same, make sure you have a recent backup before you do a change like this, just in case.
