I believe there are no plans to port Kotlin’s null safety to Java, which is possibly the biggest advantage Kotlin has right now. Once you’ve tried it there is no way you would want to live without it.
If this is the "biggest advantage Kotlin has right now" then it doesn't have much added value... Would you rewrite existing code base with millions of lines of code just to get null safety? If THIS is the biggest advantage of Kotlin, then Kotlin is on the same path as Groovy was a decade ago...
In order to replace Java and it's ecosystem one would need to offer MUCH MORE than just null safety...
It would depend on the state of that codebase, really. If it’s well tested and does its job well, I wouldn’t bother rewriting it at all.
But certainly if I were to add new features to an existing Java codebase I’d use Kotlin. The interop between the two is fantastic and for me the null safety alone makes it worthwhile.
In order to replace Java and it's ecosystem one would need to offer MUCH MORE than just null safety...