High level steps to migrate connection server to 2019 OS will be as follow:

> Take a snapshot of all CS, composer server including composer DB backup and connection server ADAM database VMware Knowledge Base

> Install windows 2019 CS as a replica to existing 2012 server.

> Once installation is finished, make sure replication is working fine between both servers by running repadmin command. This is critical before moving forward. VMware Knowledge Base

> Do the same for remaining 2019 servers.

> Decommission existing 2012 servers following VMware Knowledge Base . This should done as early as possible otherwise ADAM database may get corrupt since 2012 and 2019 server will be using different AD LDS schema and keeping both in replication for longer run may cause conflicts.

> After this you can migrate composer to 2019 server as below:

• On new composer server, prepare the ODBC connection and create a system DSN to point to view composer DB.
• On the old server, stop view composer service.
• Install view composer on new server and point it to the ODBC.
• Once installation is finished , login to view admin page.
  Navigate to View configuration > Servers > Edit > View composer >Edit and put the new server FQDN name and reauthenticate with user name and password > Click on 'Verify server information'    > Ok.
• Test the provisioning now.

In case of any confusion or doubt, please contact vmware support.