Passwords are chosen at install time, so you need to make sure you pick the same passwords on the new 64-bit server as were used on the 32-bit server, so that they work for all transfered data. The one's I'm referring to are the passwords used to connect to LDAP (the ldap root password, ldap admin password, and the amavis/postfix/nginx passwords). You probably want to make sure you use the same zimbra admin password for the mailstore too.
And no, you cannot necessarily import data from a newer ZCS LDAP installation into an old one, as newer ones sometimes have new attributes that are not present in older releases.
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