This is basically how Firebase is designed and it drives me nuts. I've freelanced with several teams, primarily comprised of front-end devs, who decided to use Firebase for their product. When they first told me they query the database directly from both their website and their mobile app, I immediately was like "so...what happens if you need to change the structure of the database"?
Crickets, really? It's very obvious how to do as recommended by Google - you use a Cloud Function (aka Cloud Functions for Firebase) to do a "migration", while simultaneously handling both cases in client code until this is done.
You must have been working with a bunch of amateurs if such an obvious solution (even without reading the documentation) didn't come to mind apparently by you or your clients (since you're the freelancer). This particular problem isn't even Firebase specific - you have to do this for any store that doesn't strictly have a schema.
This issue you're describing isn't even in the top 5 biggest issues about using Firebase.
As I stated in my comment, I was working with front end developers, who had little-to-no experience working with backends and hadn’t considered migrations. That documentation really didn’t help because it’s such a laborious and error-prone process, especially when dealing with mobile applications that live on users devices. My implied point was that an API layer resolves the need for any of that bullshit to begin with.
And yeah, there are tons of other issues with Firebase, but this was the first warning siren that went off in my mind. I hate Firebase.
Crickets.