The only way to retain their current greetings, it's to backup the greeting on a folder but this should be manually, you will need to go to each greeting page and right click on the media bar and save the greeting to a folder.
Unfortunately each account has it's own properties associated to unity, so there's no way to change the properties of the old unity user for the new AD user.
You will need to re-import again the user but with the new AD account.
I was going to tell you to use COBRA, but there are not many options on the backup - COBRAS gets the entire directory structure for all objects every time it does a backup. You can choose if you want to include voce names, greetings, messages or distribution list members if you like, but all basic object data outside of that is included in every backup to provide COBRAS restore with a complete object map to use during the restore process.
That means object (users), so as I told you the only way it's to do backup the greetings to each user one by one
HTH
if this helps, pls rate
Viri