OMD-Labs 3.x comes with some changes which might break existing setups.
A few components have been removed:
Nagios 3 and Icinga 1 will be automatically migrated to Naemon. Configuration should mostly be compatible.
If you made manual changes to your etc/nagios/nagios.cfg or below etc/nagios/nagios.d then you have
manually port those changes to Naemon.
The plugins will now be installed into lib/monitoring-plugins instead of lib/nagios/plugins. The resource.cfg will be changed automatically and there will be a symlink from the old location. So nothing should break.
Local plugins go into local/lib/monitoring-plugins and there is also a symlink from the old location.
The logos location has been changed from share/nagios/htdocs/images/logos/ to share/logos/. There will be a symbolic link from the old location.
Same applies to the local folder which has been moved from local/share/nagios/htdocs/images/logos/ to local/share/logos/.
OMD-Labs 3.x comes with a new Ansible version 2.7 which might break existing playbooks. Previous version was 2.3.
There is a porting guide available on ansible.com.