I did a migration from 32 to 64bit server, and everything seemed to have gone fine.
Everything about the Zimbra server. The samba setup wasn't so lucky.

After fixing some issues with ldap_root_password key, I'm trying to solve the problems with pam: it doesn't seem to get users from ldap!
I copied all config files from the other server: pam.d/common-*, nsswitch.conf, pam and nss config, links...
Everything look the same as on the old server!
From IRC help, I tried verbosing ldap with command
zmlocalconfig -e ldap_common_loglevel=16640
, but in zimbra.log I still don't see bind errors.

Another issue I probably have is that when I issue some samba commands I get warnings, like:
# net getlocalsid
[2010/03/15 00:20:32, 0] lib/smbldap_util.c:smbldap_search_domain_info(286)
  smbldap_search_domain_info: Adding domain info for MYDOMAIN failed with NT_STATUS_UNSUCCESSFUL
SID for domain PC0 is: S-1-5-21-134108736-853719719-3269453706
I guess this happens because I restored LDAP db of old server...

I followed the steps at Network Edition: Moving from 32-bit to 64-bit Server - Zimbra :: Wiki. I did the modifications to LDAP db specified in UNIX and Windows Accounts in Zimbra LDAP and Zimbra Admin UI 6.0 - Zimbra :: Wiki before issuing the ldap db restore, otherwise it would have had errors.

Now I guess i'd better have ignored those errors and reinstalled the zimbra_posix and zimbra_samba extensions and modifications later on...
If anyone has ideas, you're welcome. Otherwise I'll try this other approach.