A disruptive upgrade can be performed on any HA pair, but downtime is required.
In a disruptive upgrade, downtime is required because the HA configuration is disabled and each node is updated. When the HA configuration is disabled, each node behaves as a single-node storage system; in other words, system services associated with the node are interrupted for as long as it takes the system to reboot.
You can also complete other maintenance tasks, such as system firmware and hardware, as part of the disruptive upgrade. These can also take place when the HA pair is disabled.
Although nondisruptive upgrade requirements are not mandatory for disruptive upgrades (when downtime is scheduled), it is a best practice to follow the NDU preparatory procedures for all upgrades to ensure system health before and after the upgrade.