How To Migrate ISPConfig 2, ISPConfig 3.x, Confixx or Plesk to ISPConfig 3.1 (single server)
This post is from 2018. Versions, commands and links in it may be outdated. The current release is ISPConfig 3.3.2p1.
A new tutorial that shows how to migrate an ISPConfig 3 single server setup to a new server by using the ISPConfig Migration Tool is availble at HowtoForge.