I recently moved to a new server, this time with postfix, the old one was sendmail based.
After the restore process, that goes fine in virtualmin restore panel, i had 2 problems.
1) i have changed in the new server the default domain subdir to $DOM, virtualmin restore the backups in the correct place, but in the virtualhost pnp.ini still have the path for "tmp & session directory" to the old position. I had to correct them manually for each virtualhost.
2) to make simple the migration from sendmail to postfix i had to change in postfix & dovecot the default Maildir to "mail" becouse the restore proces will not recognise the old "mail" dir and add a new "Maildir".
Sorry for my rusty english. Kind regards.