Migrate Oracle 12c to 18c in AWS RDS
I was able to do this and it was a very smooth process at the end.

Just took a snapshot of the current 12c RDS instance and restore it. Restoration creates a new 12c RDS instance, but have to be careful about the instance type.

After all I upgraded it to 18c with a one click as they say in the documents. 

The schema, users, passwords were all there in the new RDS as the old one. Did not require any kind of Oracle administrator skill.