Depends on where your users were stored with exim? If you used PAM you are lucky because Postfix will be able to use it as a backend too. If users are stored in LDAP you will have to adapt your Postfix configuration to the exim schema. If they are in a SQL-DB you will have to manually port the SQl-statements for Postfix. If your users are in plain-text file chances are that you can also use it directly in Postfix as it also supports such files.
As for the Maildirs: Neither Postfix nor Exim deal directly with them. Usually there is a MDA like procmail/maildrop/deliver depending on what kind of IMAP4/POP3 server you are using.