Convert qmail - maildir running server to postfix

I have an old mailer server, where are hosted several hundred of email domains. Size of all maiboxes is around 150GB. The software is qmail, courier-imap, spamassasin. I need to migrate it to new server based on ISP Config 3 / with postfix, courier-imap and I prefere to left it on maildir base. Feel free to ask and clarify prerequisities. Send me me an overview how will you do it. The server must not have long outages. Users must be notified before migration, DNS change must be done as migration step.