From the OP quoted article:
"While Windows 8 and Windows RT can both run so-called “Modern” apps (formerly known as “Metro” apps) designed for touch-sensitive screens, even those apps must be converted, or ported, to Windows RT, and not all Modern apps appearing in the Windows app store have been ported over, compounding compatibility issues."
If you write a .NET/XAML app, by default it will be compiled to a CPU neutral IL that runs on all architectures (you can disable this and target a specific CPU architecture).
If you write a C++/XAML app, by default Visual Studio will compile different versions of the app for each architecture.
For the vast majority of Win8 apps, "converting" to ARM is just a cross-compile. It is only when you are doing very specialized things that you actually need to worry about the CPU architecture your app will be running on.