We're talking about an html GUI right? That's not herculean to port between languages.
It is a huge effort even migrating from one PHP framework to another.
Coming back to your point, there is no single “page” to be migrated one at a time.
Is there a reason one URL at a time wouldn't work?
You can always take a look at the UI of a PAN device and use your browser in developer mode to see how it works and understand the concept of a web application.
I do not have a PAN device.
> it is not "one URL"
I never implied it was.
> it is an entire application, it does not have distinct "pages"
> the concept of a web application
Those are usually written in javascript and the client part could stay the same. If anything that kind of design makes it easier to replace the backend one URL at a time becasue the responses from the backend are simpler.
If they have something where the UI state is kept on the server and is relayed to the browser, that would be difficult to do anything with. But that's an especially terrible design that causes other problems, so if you're accusing them of having a design that bad please be explicit.