Updating the Windows Server Operating System in a Disaster Recovery configuration can be performed without destruction of the DR pair. Cross-Operating System support within a DR pair is supported to allow you to step through the upgrades without a major outage.
Validate Disaster Recovery Health prior to beginning any OS upgrades.
Ensure the Primary and Secondary Disaster Recovery units are both at an equal and current patch level for Windows Operating System and SQL Server.
Upgrade the Operating System on the Secondary unit in the Disaster Recovery pair (non-active).
Apply latest Windows and SQL patches to Secondary unit to get to latest patch level for the new OS version.
Verify .NET 4.7+ is installed and patched on the Secondary unit.
Perform a Planned Disaster Recovery Failover from Primary to Secondary, making the recently upgraded server active.
Upgrade the Operating System on the Primary unit in the disaster recovery pair (non-active following failover).
Apply latest Windows and SQL patches to Primary unit to get to latest patch level for the new OS version
Verify .NET 4.7+ is installed and patched on the Primary unit.
Perform a Planned Disaster Recovery Failover from Secondary to Primary, returning the DR pair to a “normal” state.
Verify both hosts have a matching local security policy for Encryption types allowed for Kerberos to ensure AD based authentication will function. You can check this under Local Security Policy > Network Security: Configure encryption types allowed for Kerberos.