If you write an HTML/JS, it will always run on all architectures.

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.