Ok, your apps getting a huge success you want to update, expand your app. Let say going to version 2.0.
Meanwhile, you are developing the 2.0 version, think about whats for people using version 1.x.
First of all the important for the backend to support the legacy app gracefully, we want user to update to the latest app but not all people are yet to make move yet, so gracefully, do something nessecary to make sure previous user still be able to not having bad UX, such crashes. Tell user to update it.
On the backend side, we will use versioning to maintain old and new API. It looks like this https://myawesomeservice.com/api/v2/newCall. So the old app will use v1. This way to help to avoid regression to the old API.
Another thing push notifications. When we already have actual users on the market, we cannot do blasting to all users when testing the push notifications, we have to test it for tester only like ourselves. The new push notification handling also may be different from the old one, so bear in mind how to do it properly.
All in all, we want to make sure the old users understand they need to update the app to work properly as the business is growing.