We try to be compatible with the configuration files of the current Nagios 3.x versions so there should be very little you have to do to upgrade from Nagios 3.x to Icinga 1.2. Assuming you’ve already installed Nagios from source code as described in the Nagios quickstart guide, you can install Icinga quite easily.
Please visit the docs section for the complete migration guide:

Upgrading from Nagios 3.x releases

Upgrading from Nagios 2.x releases

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