We recently needed to migrate all our sites on one physical server to another server, there were more than 200 sites, and they were all hosted with Aegir. The old server was to be decommissioned, so we had to move all of Aegir's data about the site to the new server import into a new Aegir master on the new server. We also needed to do this with as small amount of downtime as possible.
In the end we migrated all the sites with about 30 seconds of downtime each, here's how:
The setup before
For clarity, here's a simplified...