There's no direct conversion back. IntelliJ has a decompiler that produces reasonable-ish Java code based on the debug symbols, so you get things like local var

There's no direct conversion back. IntelliJ has a decompiler that produces reasonable-ish Java code based on the debug symbols, so you get things like local variable names and obviously method/class names back, but, it's a decompiler, so no code comments. And Kotlin constructs frequently require synthetic Java variables to be created that were implicit in the Kotlin, so you have to clean up the result quite a bit. However note that Kotlin and Java code can co-exist very closely in the same codebase. In the same way you can incrementally port a Java codebase to Kotlin, so too can you incrementally port Kotlin to Java. There's just no automatic tool for it like there is Java->Kotlin.