A Nagios update may install some newer packages, but doesn't do a full CentOS update.
Major upgrades would require making a Nagios backup and migrating it to the new machine(see https://assets.nagios.com/downloads/nag ... ios-XI.pdf for migration steps), but running a "yum check-update" and "yum update" can be done to keep the current OS and packages up to date. I'd recommend running it before a Nagios update.
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