Piet, IIRC, referencing legacy dotNET dlls is not currently supported in Metro style apps. You can find more information in Krzysztof's talk: http://channel9.msdn.com/Events/BUILD/BUILD2011/TOOL-930C
Scarless. The amount of porting required will depend heavily on the codebase you are discussing. Some code will just come forward with little to no changes (like JavaScript). I'm assuming you are specifically talking about .NET code though. Even with porting, it's still a lot of code you can leverage in your app rather than starting from scratch. For instance, just making the .NET code into a Metro style library rather than a full-blown class library. This is a similar situation that happens today with Silverlight and the profile of the .NET Framework you have to adhere to when developing Silverlight applications.