Maxx That's what I thought when I first looked at migrating. I had been using Outlook to export and import using PST's, but too much stuff was lost (email address history, rules etc), so with help of forums posters I drafted the plan. If you try it is a lot easier than it looks.
I only have an about 4GB of mail to move in a couple of mailboxes to migrate, but it takes me no more than about 5 mins to perform the whole migration from one server to another. Obviously, the more mailboxes you have the longer that process will take. The longest bit is the rsync to copy the actual mail directories, the rest just takes a few seconds for each part. Work in VM's (if you can) and give it a go.
Alternatively, look at moving the mail directory outside of the Grommunio server and symlink the /var/lib/grommunio directory to your actual data directory. I already do this for Grommunio Files and no longer need to move the Files data when building a new Grommunio VM/server (just put the Files data path in the Files config.php file). This needs to be planned well to avoid data loss, something I am currently playing with a test VM now I have a new NAS and 10GB network to play with. Then only the databases will need to be migrated in future server rebuilds, so even less time required to do migrations.