Comment 1 for bug 2052472

> I don't think this is correct. As far as I can tell, it is possible to
perform a series upgrade with negligible downtime by doing HA failover
between the units, e.g. pause and upgrade the two followers; once those
are restored, pause and upgrade the leader.

Unfortunately, with bionic -> focal, the series upgrade for corosync and pacemaker (the hacluster charm's payloads) can't do a rolling upgrade and keep the cluster up due to major version bumps. See https://wiki.ubuntu.com/FocalFossa/ReleaseNotes

Thus for the docs we can't assert that the series upgrade of corosync/pacemaker won't affect a rolling upgrade. It's best to assume that there will be a short outage of the control plane service for that upgrade.