I am upgrading from 6.6.1 to 6.8.3. I have dedicated master, coordination and index nodes. I upgraded only one coordination node and found that in the monitoring page the node was not showing up. From the logs, it seems that the node joined the cluster. Initially, I was unsure of that too but I discussed in the Elasticseach forum for confirmation.
It seems the node count on the monitoring page appears to be the same but when I search for the coordination nodes, the upgraded node does not show up in the list.