A bit of history for the context
After running a Zimbra mail server in a 500Gb Virtual Machine, for about 4 years, the server started feeling a bit crowded and with #df -h reporting less than 50Gb of space left, it was time to move to a larger machine.
The version I was (and still am) running is the open source version, there are no migration tools available as part of the package, although you can find plenty of tutorials on the web forums about how to rsync stuff between the old and the new server. I was not comfortable with that. For starters, there would be down time involved, but apart from that I would have to rsync between two identical servers, meaning that the new server would still need to be zimbra 8.6 and running on an identical Centos 6 machine.
There was also the fact that some time ago, the server did a very bad shutdown (due to a power failure), and the database had been corrupted, and every so often a problem or two would crop up in the logs. This became very evident when an upgrade to 8.7.1 failed miserably, and the only thing that saved the day was the backup from the previous night ! I was afraid that most likely, an rsync migration would also transfer the problematic data, and that it would be back to haunt me down the road.
Since I was going to have to go through all the aches and pains of a server migration I wanted to end up not only with more space, but also to move to a newer O.S., and a newer release, so rsync was out of the question.