I too have a large installation and would like to avoid multiple days of downtime. 
I tried this method for migrating (from a 32 bit 6.0.10 install to a 64 bit 7.1.3 install) and so far I've noticed the following:
1) My commercial SSL certificate was not migrated - maybe this is by design since it is more secure to generate a new private key on the new server and generate/sign a new CSR.
2) Not all server settings were migrated (many Global Settings, many Default Class of Service settings)
3) Importantly, the "Service host name" (under Server settings) was not transferred. I did:
/opt/zimbra/libexec/zmsetservername -n mail.domain.com
to rename it to the value of the old server.
Then
zmprov mcf zimbraSmtpHostname mail.domain.com
Then
zmupdateauthkeys
I'm close to pulling the trigger on the migration (DNS, firewall changes). I'll update again when that's settled.