Trying to migrate 32bit Zimbra Mailbox to 64bit Zimbra. You can migrate LDAP data first which including all the accounts and then migrate Mailbox which including messages, calendars ... etc

I assume you using the same version of Zimbra with both 32 bit and 64  bit. But it should be able to work for all version > 6.0

Migrate LDAP Data

On 32 bit Zimbra : 

Export LDAP data in 32bit Zimbra

su - zimbra
/opt/zimbra/libexec/zmslapcat /tmp/
chown username:username /tmp/ldap.bak

On 64 bit Zimbra:

Copy the LDAP data to 64bit Zimbra

scp -P 32 <username>@<64bit zimbra>:/tmp/ldap.bak /tmp/ldap.bak

Replace the old hostname with the new one

sed -i 's/<32bit zimbra name>/<64bit zimbra name>/g' /tmp/ldap.bak

Backup LDAP passwords

su - zimbra
zmlocalconfig -s | grep password > /opt/zimbra/backup/zmlocalconfig-pass.txt
zmcontrol stop

Backup LDAP database and then empty it

mv /opt/zimbra/data/ldap/hdb/ /opt/zimbra/data/ldap/hdb.backup/
mkdir -p /opt/zimbra/data/ldap/hdb/db/
mkdir -p /opt/zimbra/data/ldap/hdb/logs/
cp /opt/zimbra/data/ldap/hdb.backup/db/DB_CONFIG /opt/zimbra/data/ldap/hdb/db/.
exit

Import the LDAP database from the old environment

/opt/zimbra/openldap/sbin/slapadd -q -b '' -F /opt/zimbra/data/ldap/config -cv -l /tmp/ldap.bak > /var/log/zimbra-ldap-import.log
chown -R zimbra:zimbra /opt/zimbra/data/ldap/hdb/db/

Restore LDAP passwords

su - zimbra
zmldappasswd $(grep zimbra_ldap_password /opt/zimbra/backup/zmlocalconfig-pass.txt | cut -d= -f 2)
zmldappasswd -r $(grep ldap_root_password /opt/zimbra/backup/zmlocalconfig-pass.txt | cut -d= -f 2)
zmldappasswd -p $(grep ldap_postfix_password /opt/zimbra/backup/zmlocalconfig-pass.txt | cut -d= -f 2)
zmldappasswd -a $(grep ldap_amavis_password /opt/zimbra/backup/zmlocalconfig-pass.txt | cut -d= -f 2)
zmldappasswd -l $(grep ldap_replication_password /opt/zimbra/backup/zmlocalconfig-pass.txt | cut -d= -f 2)
zmldappasswd -n $(grep ldap_nginx_password /opt/zimbra/backup/zmlocalconfig-pass.txt | cut -d= -f 2)
zmcontrol start

Migrate Mailbox Data

Here we write bash script to use the zimbra export and import tool.

On 32 bit Zimbra : 

Mailbox Export Script

#!/bin/bash
### START CONFIGURATION ###
DIR="/zimbra-mailbox-backup";
### END OF CONFIGURATION ###

USERS=`su - zimbra -c 'zmprov -l gaa'`;
DATE=`date +%Y%m%d`;

if [ ! -d $DIR ]; then mkdir $DIR; chown zimbra:zimbra $DIR; fi

for ACCOUNT in $USERS; do
NAME=`echo $ACCOUNT`;
echo "Processing mailbox $NAME backup..."
su - zimbra -c "zmmailbox -z -m $ACCOUNT getRestURL '//?fmt=tgz' > $DIR/$NAME.tgz";
done

echo "Zimbra mailbox backup has been completed successfully."

On 64 bit Zimbra : 

Mailbox Import Script

#!/bin/bash
### START CONFIGURATION, CHANGE THE FOLDER WITH YOUR BACKUP FOLDER ###
DIR="/tmp/zimbra-mailbox-backup";
### END OF CONFIGURATION ###

clear

echo "Retrieve zimbra user name..."

USERS=`su - zimbra -c 'zmprov -l gaa'`;

for ACCOUNT in $USERS; do
NAME=`echo $ACCOUNT`;
echo "Restoring $NAME mailbox..."
su - zimbra -c "zmmailbox -z -m $NAME postRestURL '//?fmt=tgz&resolve=reset' $DIR/$NAME.tgz";
done
echo "All mailbox has been restored successfully"