You are looking to retrieve the private key and password for the current APK in order to republish your Android app using a different app builder than BiznessApps CMS.
Our Android apps need to communicate directly with BiznessApps backend to function, thus migrating to another app-builder requires a completely new app to be created in the Google Play Store. As explained in our Transferring Apps from Another App Builder to BiznessApps article, the new app cannot be pushed as an update from the new App Builder, so once the new app is created, users should manually download the newly developed app to their devices.
As a potential alternative, you may check with Google if the App Signing key can be refreshed to use the key for the newly built app. If that is the case, the old app could be replaced under the same listing on the Google Play Store, and the next update on the Android app users will replace the app completely. Again, this possibility can only be confirmed by the Google Play Console Support.
If the above is not possible and you decide to publish a new app, you can send Push Notification from the BiznessApps CMS to all the existing users once the new app is listed on Google Play Store and maximize the likelihood of current users migrating to the new app. Please see the article Sending Push Notifications for more information.