Can existing apps (both web and desktop apps) using Silverlight run on Windows 8? How about Windows RT?
If it’s not possible, what would be the path of least resistance to make it work? (Viz., how to get it working on Windows 8/RT, not caring about whether it’ll work on Windows 9 or not.)
Existing Silverlight applications can run in the Windows 8 Desktop, but cannot run as part of a Windows Store application. Only Windows Store applications can run on Windows RT (with exception of Office apps delivered by Microsoft).
Path of least resistance would be leveraging your assets – it’s C#/VB and XAML still – within the Windows RT framework.
You might check out this Deep Fried Bytes episode, and there are other blog posts as well to give you some guidance. There’s also an article on the Dev Center focused on Windows Phone 7 Silverlight to Windows 8, but much should be application to your scenario as well.