Hi all,
Depending on the direction you are coming there is a slight anomaly with directions between Gare de Lyon and Gare de Paris-Austerlitz. Even Google Maps sometimes gets it wrong. The best way is to just walk it!
The pedestrian route is totally doable in less than 10 minutes between the two and this makes far more sense than trying to change on public transport.
I have added a map for to show you and no matter how I try Google maps brings you in a roundabout way to do what is actually easy, direct and safe.
Hope this helps!
Mick
https://drive.google.com/a/catalamoran.com/open?id=141ZiBZa8gNYe53SPCmOLhHU_ZnqYXtV9&usp=sharing