Steve Stedman gives us the proper order for upgrading SQL Server when you’re using merge replication:
Working on a recent SQL Server merge replication project we needed to update some of the servers in a merge replication scenario without upgrading all of them. Consider a merge replication setup with a publisher, a distributor and 2 or more subscribers all on the same version of SQL Server, and you need to upgrade the SQL Server version on the subscriber to a newer version like SQL Server 2019.
Before doing any type of upgrade, I wanted to confirm that things would or would not work. First checking some Microsoft documentation it appears that replication from a SQL 2012, SQL 2014, SQL 2016, or any older version of a publisher is not supported to a subscriber running on SQL Server 2019. Or more specifically the subscriber needs to be on the same ore older version than the publisher.
Read on for a demo, as well as an interesting caveat.