In theory it’s possible to swap in a new UX, as the server side part is an API and the client consumes it. You could switch out the Ember client for another one using the same APIs.
In practice this project will likely never get done if you want to maintain the complete functionality of the application. If you want to drop tons of features, sure, you could get something working pretty quickly.
As an example, our next release of Discourse will include a new rendering engine for topics. It still uses ember for all the data models and actions, but the rendering is a virtual DOM similar to React.
The project took me 1 month of solid development before it was in alpha, then it took me another 3 weeks to sort out bugs and get all the plugins working to what I consider a beta. And bear in mind, I’ve been working on this codebase for almost 4 years now and am very familiar with how it works! A developer who didn’t would likely take much longer.