Oracle database 12c upgrade to 19c
Upgrading 12c to 18c or 19c should not be a problem. I don't think that 19c is that different from 12c. The dbua tool should be able to do the upgrade. You still need to test with a test database. But running the dbua tool should work. Going from pre-12c to 19c is a much bigger issue depending on what features are used.