From the procedure I learned at the last phase of upgrade,  we should change compatibility level for data center/cluster, respectively.

But may I know what exactly the settings we are changing - a config in a database or in a file? Actually my question is, are these changes can lead to any service downtime for "guest VM" or "Storage domain", "hypervisors"?

Because lately my RHV 4.2 suffered a few network disconnections and that causes hosts to be fenced forcefully. 

I hope I can collect more info here to prepare for 4.2->4.3 upgrade.