Assume the following situation:

You changed something on a service and the performance data format has changed as well. PNP4Nagios graphs that belong to the changed service will not be changed automatically. You have to remove the old XML file that belongs to the changed service and then re-start your OMD site completely like this:

OMD[mysite]:~$ rm var/pnp4nagios/perfdata/webserver35/Apache_127.0.0.1_80_Status.xml
OMD[mysite]:~$ omd restart

In a few minutes the XML file should be re-generated and graphs belonging to new performance data counters should appear. If NOT, I’m afraid that you have to remove all RRD-files that belong to the service (you can recognize it on the file name) and to hope that they will be re-created properly. In this case all the server statistic will be lost (you have been warned!).