This is a little editorial:
It is good admin etiquette to backup. In the open source edition, it doesn't include and automated backup process, but you CAN backup your files!
All information is most likely located in the /opt/zimbra folder. If you stop zimbra, and simply tar up the folder stick it in a different location, then you are good to go!
I cannot stress enough to our valued users, TAKE TIME TO BACKUP!!!
Let's take a failed upgrade as an example.
If you upgrade from 4.0.x to 4.5 and it fails (for any reason), it may be possible to install your 4.0.x over your failed upgrade and get it working. HOWEVER, you now *may* have tables in the db that are not active, and may cause you future upgrade headaches.
We love helping, and providing the very best support in the open source community, but it's important that you keep your info safe.
So, to sum it up: before you perform any sysadmin maintenance like upgrades, injections, or anything else, take time to backup.
You may be down a little longer, but you'll be better in the end.