Thanks (again) for the quick reply!
Usually you just need to change the domain on the GitHub side.
Ah, bummer. What if the application has already been removed? Looks like I can at least hit the “Forgot password” and make a new password, assuming that’s because of this:
We always associate logins based on email
But I can’t hit the login with GitHub anymore as it’s still giving me the error.
We do mention this in the howto on changing your domain name
Found it. Linking to it for reference.