I have made some changes to the Android app signing process (and app signing key) in order to make the system more stable over the long term. Unfortunately, in the near term this means that apps signed using the old key can’t be updated with new builds.
In order to post an update to previous builds that are live on Google Play, you’ll have to make a new app (with new app id) and disable the previous one. If this is a major problem for anyone, let me know - I can help make a build with the previous key if it is really needed.