Seems like the migration from 2012 to 2019 is pretty simple. Introduce 2019 DCs to the domain and demote 2012 servers. My question is that when I introduce the first 2019 server do I have to do anything to the server running Azure AD Connector since the schema is upgraded etc ? Upgrade AAD or refresh or anything ?
If you do not do anything, sync will work just fine. 2019 introduce one and only one attribute preferredDataLocation and this is useful for AADConnect sync. So, with new 2019 Schema, you can change the mapping of preferredDataLocation attribute and can leverage it.