The new version of Unread, coming tomorrow (Thursday 23rd) will be a free download. Users can read 50 articles for free and when those are read, 3 more/day until they unlock unlimited articles for $4.99. Previous owners of Unread will be looked after. We have implemented a ‘handshake’ where our app asks Jared’s app a question that only it can answer. This exchange reveals a free in-app purchase to unlock the app. It’s important that current owners update to 1.3.2 of Jared’s Unread now because this adds the code to perform that handshake. - SuperTop
In order to get the update for free, you need to have yesterday's update running on your iOS device. So you better update the original app right new, before it's removed from the App Store.
Migrating apps sounds complex.