This is from a Microsoft employee:

That is correct. In general the move from x86 to ARM is just changing the target and recompiling. So long as your app gracefully handles lower end hardware you should be fine.

Once ARM systems are available you will want to test on them, but I wouldn't expect significant problems.
Sounds like a recompile is needed for C++, but if the app is coded correctly, it really should be as simple as a compiler switch and recompile. Something that could even be autmated with MSBuild.