[CentOS] warning for nis users and 5.3 upgrade

>Folks, be sure to do an updatedb and then locate rpmsave and rpmnew
>after upgrading the system. Then you can make sure your local changes
>get propagated into the updated system.
 
If the case is a file gets overwritten, and not a case of a file gets
copied to a new one called *.rpm{save|new} during an upgrade, what
exactly does the above do?