Hi
Your migration system works awesomely, and top marks on that
Based on my own experience of migrating Zimbra from 6 to 8 from one server to another I did had an idea about a feature that you could add to your migration software though that I think is probably unique (as I cannot find any other service to provide this),
The idea is in essence is to add more control over the switch over point by adding the ability to synchronise the two systems
In order to make the process as stress-free as possible on single server installs (at least) would it be possible to add some sort of server-client feature of the zextras migration tool to forward any incomming mail to the "old" server over to the "new" server so as to keep them in sync ready for the switch over.
If this is possible then it would allow you to build up the new system, check it works fine, then at the migration point shut down the first system and alter internal dns settings to point to the new system (e.g. mail.domain.com now internally resolves to server B not server A)
I appreciate I may not have conveyed this very coherently, but hopefully you can see where I'm headed with the idea 
Feel free to say "you fool, that won't work" but the idea of being able to control the migration process better by allowing the servers to automatically sync is pretty neat and similar to a process we use in our own web based telemetry system when we migrate customers from server to server whilst maintaining a set of remote outstations