Yup, a new release will install over the top of a prior release, and you don't loose users/mail.
But there are overwrites for obvious reasons (think about it-there really is no other way to add new content/correct errors without some massive logic).
-Do a complete backup of /opt/zimbra and any other symlinked/mounted directories. (best to use a method that will keep file ownership/permissions intact)
-grab the new version and ./install.sh (it will find your current install)
-There are a lot of overwrites for new content/schema's, to get a fresh base, etc; after you upgrade compare files. (obviously the greater the gap in versions the more you will notice)
-Things set via the admin console (with the exception of zimlets) are preserved. As well as stuff set via
Zimbra CLI Command's
Really you'll find that it becomes easy to do-you can always make scripts to copy back in settings/skins etc. But everyone has a different config-it would be too much logic to accommodate all the possibilities. Keep us posted-and of course ask questions
