I see this is from the ionic people - What are the goals for this vs ionic? Is this supposed to eventually replace it? Will it still be pushing Angular on us, or is it possible to use any JS framework?
This is not a replacement for Ionic! Ionic provides a cross-platform UI layer for mobile web apps. Capacitor is a replacement for Cordova which manages the native SDK interaction and native app project management/tooling side of your app.
Once we integrate it into the Ionic CLI, you won't really have to think about which part is Ionic UI and which part is Capacitor.
Once all your plugins are supported in one way or another (Capacitor plugin or compat for Cordova plugin) you should just be able to install Capacitor in your project and see if it all works. Required code modifications in your app should be limited to the interaction with native.
Once we integrate it into the Ionic CLI, you won't really have to think about which part is Ionic UI and which part is Capacitor.