I have an unpublished app on Google Play which I want to update for my existing users, but not let new users download it.
The Google Play documentation seems to be saying that this can be done by simply uploading an updated apk (and not re-publishing the app):
Unpublished Apps
Existing app users will still be able to receive app updates, even if you unpublish your app. If you don't want new users to find and download your app but still want to provide updates to your existing users, set your app to Unpublished in the Developer Console. https://support.google.com/googleplay/android-developer/answer/113476?hl=en
But it seems unintuitive that an "unpublished" application would work that way. If the documentation is correct, how does Google Play tell the difference between an 'existing' user and a 'new' user?