change postfix, dovecot and roundcube server to ISPCONFIG

1) Create the mailboxes on the ispconfig server. E.g. by creating them manually or by using the remote api or by using this tool https://git.ispconfig.org/ispconfig/Modules/tree/master/mailbox_import_from_csv
2) Move the contents of the existing maildirs to the newly created maildir folders in ispconfig. Maildirs in ISPConfig can be found here: /var/vmail/domain.tld/user/Maildir/

 

If you just want contacts, you can do that in the roundcube interface from the address book, export to a file, then import on the new install. If you want more than just contacts, try https://github.com/JaimeObregon/export-roundcube-sql (I have not tried that yet, but intend to soon).