I'm not sure of the reasons for everyone to move to the Appliance (though I've read you seem to have some various reasons for doing so), but I would certainly suggest holding off for now. We do not yet have verified documentation on the process for completing such a move. One thing that could potentially help you would be making sure you're at least on ZCS 7.1.4 before doing so. If you're on 7.1.4 or above, in theory you should be able to bring up a new appliance running zcs8 as a mailstore, and then using zmmboxmove to move your mailboxes to the new platform. Once that's done, it gets a bit tricky. Since you need to then eventually shut down your old system, you're going to need to be able to transfer mta, ldap, and potentially proxy to the new host. In order to do that, you'll need to upgrade the old box to 8 first, and then bring up the appliance as a replica and mta (and possibly proxy) server as well. Additionally, it should be noted that you're going to hit Bug 77669 and Bug 77597, and there's no way around that. All single-instance storage will be lost, with no current ability to reclaim it. What that does to your storage considerations is not easily determined, but if you have a lot of single-instance storage, that could be a considerable hit on disk usage.
Documentation for these procedures is currently being worked on, but I don't yet know when it will be available. I would suggest holding off on any moves to Appliance until that has been released though, since you could run into a big mess if you start and end up stuck somewhere along the line.