I need to move a domain from a ISPConfig to another, I've seen the web and mail directories so making tarballs of their /var/vmail/domainname and /var/www/clientX/webX directories shouldn't be a problem but which tables and config files should I look at to make a "transparent" transfer? (no mail/ftp account password changes)
Moving a domain manually by changing the database and confi files is very error prone. the better approach is to make a tar.gz of the web folder and other files that you might have uploaded for this domain, then delete the website ad create it again on the new server and then restore the data from the tar.gz.
That's part of the original plan, but I still need to transfer mail account passwords to make it go smoothly. I can issue new passwords to ftp users (1 account per domain mostly), but there are domains with 20+ mail accounts per domain and issuing new passwords is not the best option for me.
You acn import the mailbox settings with the function Tools > Import > Ispconfig email. Afterwards move over the whole file tree /var/vmail/domain.tld to the new server.