I'm also a bit unclear about updating to 4.3 cluster level.  I have a 3 host HCI setup which I just updated to 4.3 manually.  I upgraded engine to 4.3 then upgraded each of the three hosts to ovirt node 4.3.  I still need to change cluster/data centre compatibility level to 4.3.  If I try to change cluster compatibility level to 4.3 it tells me that all hosts need to be in maintenance to do so (does this mean I need to shut down all VMs and put each host in maintenance?)