I created a clean cluster with all services and enabled NameNode HA in preparation for an upgrade. I distributed my new HDP version to all 20 hosts, but some hosts failed because of repository issues.
I went to each failed host and manually retried the installation, which seems to work. When I went to Manage Versions, it showed my new stack with only 18 out of 20 hosts upgraded. I had to wait about 10 seconds for it to show me all 20 hosts had the correct stack version.
At this time, the "Install" button changed to "Upgrade". I hit Upgrade, but the second I hit it all 20 hosts went down to 0 for "Installed" and instead of an upgrade, Ambari tried to re-install the stack on all hosts that already had it.
The installation went very quickly since all hosts already had the bits.
I noticed that, in general, the Stacks and Versions section was slow to update, sometimes showing me information from several minutes ago. For example, I had 14 hosts installed, but it only said 4 were. As I waited, it eventually caught up.