Edit underlying model of application?

I just started playing with the application builder. First off, it is very cool! I think the potential here is huge.

However, it appears to me that once you start programming an application there is no way to go back and edit the model it is based on. Is this correct?

So if I spend hours (or days) fine tuning the application interface, then decide I would like to add an additional post-processing feature (or something much more complicated like an additional physics node), it seems I would have to start the app development over from scratch.

If that's the way it is, so be it. Just want to make sure I'm not missing something. Thanks.