As changes are made, I find myself wanting to use some new features or make updates that are only behind the scenes, but not wanting to break my original app.
If I duplicate my app, make the changes I want, then use the URL from my original app to point at the new version - will that automatically update any users who already have in installed?
Yes. I think one of the only things that would not automatically update is the app icon on the home page of the phone. It would have to be un-installed and re-installed. But why don’t you do some simple testing on another simple app to see how it works.
You are correct there. But it was implied if you read between the lines so to speak 
I added it for those who can’t.