==== Required information ====
- iRedMail version (check /etc/iredmail-release):   0.9.3
- Linux/BSD distribution name and version:    Centos 6.7
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):  MySQL
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro?  Yes version prior to 2.3.0
- Related log if you're reporting an issue:   N/A
====

Read this article... Upgrade iRedMail from 0.9.3 to 0.9.4
http://www.iredmail.org/docs/upgrade.ir … 0.9.4.html

It has nothing about actual mail packages like Postfix, Dovecot, etc. and appears to be no particular scripts for upgrade.
Essentially is upgrade:
   iRedAPD
   iRedAdmin-Pro 2.3.1
   Upgrade Roundcube
   Confirm cron job exists and update the iredmail-release file

Is there anything missing in these steps? 

Are the only database modifications taking place in the iRedAPD upgrade?