To make "change the app" more clear: Riot had to change its package name in order to be able to sign the app with a new certificate. This results in a new app entry in Google Play.
@fdroidorg could theoretically keep the original package name with upcoming versions, but this would introduce a maintenance overhead on the already overloaded F-Droid maintainers. Therefore, F-Droid aims to be as close to upstream as possible which means in this case switching to the new package name, too.
Heads up to all #Riot users: with the recent attack on @matrix' infrastructure, it's possible that Riot's Google Play version got compromised. This doesn't affect Riot's F-Droid version. Just as Riot started to do now, F-Droid has always signed all its apps on an inaccessible, offline machine. For more information, see https://riot.im/reinstall
To avoid maintenance overhead, however, it's likely to happen that F-Droid users must also change the app in near future. Still, there's no need to act now.