Let me say first that if I have a working application, I usually leave it alone. That said, if I wanted to port it to another language, I would start by making sure I had a good set of test cases, and if you're trying to learn Kotlin, you can write your tests (including JUnit tests) in Kotlin.
Again, the biggest advantage of Kotlin over Java is its null safety. In addition, it has excellent concurrency support in the form of coroutines. Other advantages provided by the language are modern support for sequences, builders, extension methods, and more. In fact, frameworks like Spring use Kotlin to create DSLs to configure beans or applications, leading to simpler code that's still type safe.
Anything you can do in Java, though, you can do in Kotlin, so there is no reason you could not port over your app. As I say, though, if your app already works, I'd probably live with it.