Not just difficult - it becomes impossible. You can no longer develop any android app without Google's approval, just like iOS. The official emulators might not even work.
A repo is just files in a directory, so the namespace can be changed, but the whole thing stinks. Having to setup Android signing keys and needing to provide ID is not fun.
It means you won't easily be able to run builds on Google certified Android devices that aren't from "approved" people.
That's where the "prohibitively difficult" part comes in... surely they don't expect every developer on every open source app in the world to have their own app registration/package name for the same app, do they? Feels like an N * M problem, if so.
If so, then this change will likely make it illegal to distribute APKs of GPLv3 software, since the recipient couldn't run their modified version.
And thus accelerates Google's push away from APKs, preferring instead for all developers to embrace their proprietary App Bundle format. Complete with ad hoc signing performed by the Google Play store at time of download. The bundle is also customized to the device, meaning an .aab file ripped off a device won't necessarily be loadable on another device since it could have different configurations/hardware that happen to limit it.
I think anyone who works as a dev knew this was Google's endgame the moment they started circling the wagons with the app bundle stuff. It was already getting weird before that, but it was uncharacteristically out of step with historic Android.
Doesn't this make it prohibitively difficult to do local builds of open source projects? It's been a long time since I've done this, but my recollection was that the process to do this was essentially you would build someone else's (the project's) package/namespace up through signing, but sign it locally with your own dev keys. A glance at the docs they've shared makes it sound like the package name essentially gets bound to an identity and you then can't sign it with another key. Am a I misremembering and/or has something changed in this process? Am I missing something?