There's absolutely no reason to pay for Oracle's JDK unless you want Oracle's support.
It was probably intentional, but Oracle really fucked up the marketing on this change and as a result a bunch of FUD has been spread throughout the Java ecosystem.
I have never seen an issue from switching Java SE->OpenJDK. We did the whole company in one day without outages. I'm convinced that incompatiblies don't exist in practice.
To answer the OP, we didn't rewrite anything, just switched to a no-cost JRE. And since we did it at the same time of the Java 8 -> 11 switch, there were no extra dev costs.
”Today we’re announcing another big step: Android development will become increasingly Kotlin-first. Many new Jetpack APIs and features will be offered first in Kotlin. If you’re starting a new project, you should write it in Kotlin”
“Just use Kotlin” doesn’t help for the licensing issue, though, if you keep using Oracle’s JVM.
My good friend is a senior Oracle sales person and he felt like they stabbed him in the back when they did it. He spent years developing good relationships and now this.
the tl;dr is that there are multiple OpenJDK vendors now, Oracle just happens to charge for their JDK distribution, but you can choose from several vendors (Azul, Red Hat, IBM, AdoptOpenJDK, Amazon Correto, etc)