If you follow those steps in the link provided, it will show what to do:https://assistants.microsoft.com/
Essentially, you need to ensure the client URLs and certificates on the 2019 servers match the 2016, then move mailboxes, then remove the send connector on the 2016 side.
You didnt mention hybrid before, but if you are in a hybrid config with Exchange Online, then run the Hybrid Wizard and pick the 2019 server(s) as the new endpoint instead of the 2016, then you can remove the 2016 servers.
Make sire the 2019 servers have the necessary firewall ports open to external (25. 443) and DNS records are updated as well.