Sure, but they've got plenty of buffer time since their app doesn't break when the new version is released, it breaks when the old API is actually depreciated. Just because you release a new version on the App Store doesn't make clients with old versions of the app disappear.

Apps won't be updated instantaneously on devices, though. There'll have to be some backwards compatibility for a good while if the banks want to change their APIs drastically. You can't just shut down access to your customers apps.

Consider that e.g. Barclays randomly breaks their own mobile app for substantial subsets of their own customers. Over the last 4 years or so, the app has been broken for me more than 2 of them because they apparently consider it acceptable to whitelist phones based on their own whim (if you have one of the big brand phones, presumably it usually works, but e.g. even users of relatively well known brands like Huawei can often find their new phones won't work with Barclays for several months before it gets whitelisted).

In other words, many of the big banks have such a dysfunctional relationship to internet banking that customers of many of them have learned to deal with not being able to use the apps for long periods of time - it's hard to imagine that a third party will be perceptibly worse.

The bank has to deploy their client code to all of their users before they can depreciate the old API. That should give them plenty of buffer time to reverse engineer the changes.