I successfully updated our install this weekend from v4.5.9 32bit on RH4 to 64bit on RH5 (then to v5.0.1 NE) following the documentation here:
Network Edition: Moving from 32-bit to 64-bit Server - Zimbra :: Wiki
However, there's a couple of edits to that doc that should be made.
1 - Step 2a on the new server tells you to rm -rf /opt/zimbra/openldap-data/* . Don't do that - delete all contents except DB_CONFIG.
2 - This doc doesn't discuss migrating your postfix queue which should be copied over from /opt/zimbra/postfix/spool in step 5.