Hi @pdsqsql ,
Welcome to Microsoft Q&A!
Please refer to this: https://learn.microsoft.com/en-us/sql/database-engine/log-shipping/upgrading-log-shipping-to-sql-server-2016-transact-sql?view=sql-server-ver16#UpgradeSecondaries
The upgrade process involves upgrading the secondary server instances of SQL Server before upgrading the primary server instance. Always upgrade the secondary SQL Server instances first. Log shipping continues throughout the upgrade process because the upgraded secondary server instances continue to restore the log backups from primary server instance. If the primary server instance is upgraded before the secondary server instance, log shipping will fail because a backup created on a newer version of SQL Server cannot be restored on an older version of SQL Server. You can upgrade the secondary instances simultaneously or serially, but all secondary instance must be upgraded before the primary instance is upgraded to avoid a log shipping failure.
Hope this would give you some help.
Best regards,
Seeya
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