Help with old house tub drain

[Newtoncd]

You guys have helped this novice on 2 projects and saved me a bunch. I recently replaced the tub drain stop (https://terrylove.com/forums/index.php?threads/how-to-remove-tub-strainer-drain.102650/) -- thank you for the help!!

Once we started using the tub, now a different part of the pipe is leaking underneath. I want to replace completely with PVC.

I see how to remove the PVC from the cast iron, but how does the piece separate from the tub (right image0?

Thank you.

 

[Breplum]

That is called the drain shoe. The top chrome trim is unscrewed form the top side, usually with a special tool like drain dumbell.