I went through the very painful process of chaning my hostname without reinstalling zimbra. This will probably void your warranty, but it worked for me (FC4, M1):

Change your hostname using hostname. Change your hosts file and your /etc/sysconfig/network

use zmprov to change the following fields to your new hostname
zimbraLmtpAdvertisedName
zimbraServiceHostname
zimbraSmtpHostname

for example:

zmprov cs oldserver.com zimbraLmtpAdvertisedName newserver.com ....

Then, for every single user in your sytem (that's right), you have to do this:

zmprov ma user@domain.com zimbraMailHost new.domain.com

I suggest writing a shell script. To get a list of users, do this or something similar:

zmprov gaa -v | grep '# name'

Once your done with this tedious task, do a:

grep -il oldserver.com ~zimbra/conf/*

Change the hostname in any of the files returned by this command using vi or whatever.

Then shutdown your zimbra software:

zmcontrol shutdown

Start it back up:

zmcontrol startup

The only service that will start is ldap and zmmon (for the server in question, at least). This is where it gets fun. You have to manually edit the directory as follows.

First figure out your DN. From ~zimbra/openldap/bin do a:

./ldapsearch -x | grep oldservername.com

You will see a line that looks like this:

# oldhost.domain.com, servers, zimbra
dn: cn=oldhost.domain.com,cn=servers,cn=zimbra
[snip]
cn: oldhost.domain.com

This is the dn of your server. You will need to change it. To do so create a file called, for example, changes.ldif. For the above example, it would look like this:

cn=oldhost.domain.com,cn=servers,cn=zimbra
cn=newhost.domain.com

Now, grab your zimbra_ldap_password from ~zimbra/conf/localconfig.xml.

Then run the following command from your ~zimbra/openldap/bin dir:

./ldapmodrdn -h localhost -w password_From_above -D "uid=zimbra,cn=admins,cn=zimbra" -x -r -f changes.ldif

Now do a zmcontrol shutdown. Then do a ps -U zimbra and kill any zimbra-related processes that are still hanging around. If you kill perl, you should clear the .pid file here:

/opt/zimbra/zimbramon/FIFO/zm.pid

Now rebuild your SSL keys for the new hostname.

If all went well, everything should start on a zmcontrol startup

One thing I didn't address is changing the hostname on statistics data in mysql, but it doesn't seem to cause a problem, so I haven't gotten around to it yet.

Make sure to back everything up and cross your fingers. This should really be possible through zmprov or the admin console, but beta software is fun!