Once again, Daniel Lehenbauer saved my day. After my post yesterday about XBAP and EXE applications, I sent him an email asking for his opinion. He was very fast and I got the answer today. Daniel pointed me to this page, which describes pretty exactly what I wanted, and which comes to the same conclusions as I did: As long as the navigation model and the security permissions allow, switching back and forth is rather easy.

Note the use of compilation switches and of runtime checks to see if the application runs in XBAP or EXE mode. This allows to take different actions, and to optimize both modes.

I also found this presentation from TechEd 06, which explain very well the permission system in XBAP applications, and how to code to avoid nasty surprises during execution.

Of course all this is possible only if you design your application accordingly, and in advance. It's still unclear if we will follow this variant or not, but at least we have a more solid basis to move forward.