So this is the first time I am going to send an update for my app and I don\'t know about what actully happens when an app is updated via google-play,
Here are some ques
You may be out of luck here as there is no clean way to do this with an already installed app. If you have in-app billing (and this is speculation) and Google's in-app billing system has a dynamic dashboard or API to register users, then, you can say setup a new unique key of some sort and track it through this in-app billing like system. This may not be possible though.
The second thing you can do is to look up users in your server database and create this file you mention for all users and hold them in server cache (for fast access). Then in your app on first launch of this new version you can quickly get this file to user. This seems to be a good safe solution.
Good Luck, this is an interesting problem and I will be looking forward to hearing how you will solve this.
Just remembered, you can also look into push notifications and pushing this data to users. But, this assumes your app has this.