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.
@Tim_Sullivan Hello Tim,
Did you test your hypothesis of using the URL from your original app pointing at the new version of your app to automatically update any users who had previously installed the app?
Yes, I did update one of my PRO apps from one version to another by updating the URL. It was an app I built for internal use. A handful of my users had issues when I first released the app, so they were trained to delete the shortcut from their homescreen and reinstall it, requesting for a PIN if needed. I can’t say for sure it was invisible, but nobody approached me for help.
My best advice is try it with a practice app and see what happens. Good luck!
Thank you for the update Tim! I’ll follow your advice