For cases like this, it would probably be worth the PEP discussing how the transition would work - particularly if people are using foo.bar as package names right now, but the namespace version would be foo-bar (or vice versa, or whatever). I don’t think that keeping the old names and publishing new versions under the new names is a very good idea - it makes the name clutter on PyPI even worse, and adds to user confusion (should I use aws_cdk.foo or aws_cdk-foo?)
The transition discussion should also address where there’s a clash right now - with aws_cdk.official owned by AWS, and aws_cdk.impostor that’s not owned by AWS, how would transitioning to an AWS owned namespace work? Obviously the name of the non-owned package wouldn’t change. So what would change? And how would users perceive that change?