Hi! I'm planning to upgrade my server from Zimbra OSE 5 under CentOS 5.4 32 bit to Zimbra 7.0.2 under CentOS 6.3 64 bit.

I found and "old" procedure:

(1) On old server, as zimbra user, export LDAP data:
/opt/zimbra/openldap/sbin/slapcat -f /opt/zimbra/conf/slapd.conf -l /tmp/ldap.ldif
(2) On old server: shutdown Zimbra
(3) On old server: create a .tar.gz file of /opt/zimbra
(4) Transfer ldap.ldif and the /opt/zimbra tar file somewhere convenient (ie to another system on your network)
(5) Shutdown old server system. All further steps apply to your new system only.
(6) Bring up new system with same IP/hostname
(7) Install Zimbra using the appropriate packages for your new OS. Follow the Zimbra installation guide as far as pre-reqs etc, and run install.sh as if you were doing a new install. What you answer to the questions isn't important, except you should install the exact same set of packages as on your old system (I think you may be able to "install.sh -s" to avoid some of the questions?)
(8) Shut down Zimbra if the installation process started it up
(9) Transfer those .tar.gz and ldap.ldif files onto your new system
(10) In /opt/zimbra/openldap-data, delete everything except DB_CONFIG and the db and logs directories (delete the files inside those directories, just don't delete the directories themselves)
(11) As zimbra user, import your ldap.ldif, ie:
/opt/zimbra/openldap/sbin/slapadd -f /opt/zimbra/conf/slapd.conf -l /tmp/ldap.ldif
(12) As root, move openldap-data to a safe place, ie:
mv /opt/zimbra/openldap-data /tmp
(13) As root, delete your entire Zimbra directory, ie:
rm -rf /opt/zimbra
(14) Extract your .tar.gz file as /opt/zimbra
(15) As root, move that openldap-data we saved earlier back into place, ie:
rm -rf /opt/zimbra/openldap-data/*
cp -a /tmp/openldap-data/* /opt/zimbra
(16) Fix up any permissions issues in your new /opt/zimbra dir, personally I just ran /opt/zimbra/libexec/zmfixperms
(17) In your installer dir, edit util/utilfunc.sh, commenting out the section of code shown in this post:

(18) Re-run install.sh with -s option and ignore errors
(19) recopy your /opt/zimbra/.saveconfig/config.save from your old server to your new server's /opt/zimbra/.saveconfig directory
(20) ***Repeat step 11***: As zimbra user, import your ldap.ldif, ie:
/opt/zimbra/openldap/sbin/slapadd -f /opt/zimbra/conf/slapd.conf -l /tmp/ldap.ldif
(if this errors about ldap_log_level edit the slapd.conf and change @@ldap_log_level@@ to your 'zmlocalconfig |grep ldap_log_level', also you may need to stop ldap before doing this step if it is started.
(21) re-run /opt/zimbra/libexec/zmfixperms as root
(22) re-run install.sh without the -s option

Should work ?