I have a similar situation - moving from Ubuntu 16 to 18.
My thoughts are to
- bring up a new server,
- install\configure what I need
- recreate the Virtual Servers\Domains,
- copy the public_html folders,
- recreate email accounts etc.,
- use a command line program (can’t remember the name but it’s on my laptop) which copies IMAP mailboxes from one server to another
- switch the DNS records to point all Virtual Server domains to the new server’s IP.
HIH
Dibs