what about those old mails....do i need to copy them over to the new maildirs or is it possible to somehow "rewrite" the old maildirs to the new domain!? if yes, how? (we only have the free version of iredmail/iredadmin/ldap)
Adding new mail domain won't migrate mailboxes automatically, so you have to migrate them manually. For example, copy mails in old mailboxes to new mailboxes.
If you have iRedAdmin-Pro installed, you can add an alias domain for old mail domain, then all emails sent to both old and new email domains will be delivered to the same mailboxes. For example, mails sent to user@old_domain.com or user@new_domain.com will be delivered to the mailbox of user@old_domain.com. But i don't think you want to keep seeing old mail domain name. So try to migrate mailboxes manually.