I’d say it’s akin to Chrome and Chromium.
Along with the release of Chrome, Google open-sourced and released a predominant part of the source code and released it as a Chromium project, which is open source.
But Chrome has more features and deep integration with Google services and telemetry. And you have the likes of Edge which is Microsoft’s fork of Chromium with their own features. But separate enough to compete with Chrome.
So you can still keep the main source code of the app open source but keep all the proprietary on a closed source fork. Good for competition and allows the main developer to make money
The API keys for third party apps still work if the developer (or Reddit) hasn’t revoked them.
I’m not sure about an API switch over, (migrating to a new version, revoking access etc), all I can remember is billing for API credentials starts rolling from 1 July and Reddit won’t start requesting payment for a month or so (citation needed)
Please correct me (with links to facts) if I’m wrong as I’m slightly out of the loop