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Okay the way I would go about it would be as follows :-
1.Take a couple of backups !
2. Follow this document
Open Source Edition Backup Procedure - Zimbra :: Wiki
3. Take more backups of the target server
You should now have ZCS up and running on the 64bit server. It is your choice really whether you upgrade to 5.0.4 on 32bit first, or migrate your 5.0.2 install first and then upgrade. Personally I would go for the second option as it is all working currently.