There are some threads in the Migration section of the forum with good information about migrating to a different server. It depends if you're keeping the same hostname and IP, or changing them, what will happen.
It basically involves taking a mysqldump, and copying of all /var/www/virtual/ files and /var/mail/ files.
Make the new server, install same i-mscp version and configuration. Stop i-mscp_daemon.
You take the mysqldump from the old server, and import it to the new server (backup the new server first, you need the debian-sys-maint password entry). Set all domains statuses to "change". If you changed the IP address or hostname, change those in the database as well.
Start i-mscp daemon, it should start creating all the domain directories, etc.
Make sure /etc/groups has all the right users and all the /var/www/virtual directories are there, then copy over all those /var/www/virtual files and /var/www/virtual files.
That's kind of a basic overview, there are a lot of things that can go wrong, or it could go smoothly, it depends on if you have custom configurations, etc.