I'd like some advice on the following situation. We've an old Zimbra setup at the office, currently running Zimbra Network Edition 5. It's still working OK but just TOO OLD, I know! But the users are satisfied and Zimbra just works :-)

I need to migrate this setup away, since the machines on which Zimbra runs are getting too old. I want to take this situation to migrate to the new Zimbra 7.

This is our setup:
Release 5.0.16_GA_2921.UBUNTU8 UBUNTU8 NETWORK edition
3 servers (MTA, LDAP, MBOX)
25 heavy users (2GB+) mailboxes, mail, agenda, addressbook, mobile (mostly iPhones)

So how should I be doing this?
I have a possibility to setup a brand new Zimbra 7 environment (using virtualisation) and then move the users over. Or I could just upgrade the current servers and then move over. But it's just that I don't feel comfortable upgrading this old installation and have the feeling "there's no way back" when something breaks. What is a save way to do this?

Some downtime is OK (in the evening or at night) but the next morning all should be working again.

How would you handle this situation?

Thanks, Remi