Is the upgrade path from Win32 to .NET to WinRT feasible?

Question

I support a VB6 application. It's currently being replaced, module by module, with .NET (including a lot of new features as we go). The GUI is in WPF, but with almost zero coupling to the business logic, so that's not as important. I'm hoping the migration will be done by mid-2012.

I'm reading some of the news out of the MS build conference, and it looks like WinRT with Metro will be the API and GUI of choice for future development, especially if you want your application to run on mobile devices (which we will, certainly).

What kind of speed bump are we looking at when we (eventually) migrate from .NET to WinRT and Metro? It looks like C# and Xaml will be around, but WinRT is not .NET. Are the API's compatible (just a matter of changing an option in the VS build menu)? If they're not, will there be a one-way conversion tool? Will it be possible to easily maintain one application that runs under .NET/WPF on Windows XP/7 systems and simultaneously builds to a WinRT/Metro target during the time when we have all those systems in-house?