It's possible they may switch - Apple has switched processors several times in the past.
It would be unlikely to impact on software developed for the Mac. In the first instance, Apple has always provided backward compatibility for software compiled on a previous processor when a switch occurs. And secondly, Apple's developer software is already capable of targeting multiple processors – so existing code could like be rebuilt for the new processor with minimal fuss.
The biggest hot would be to software like Parallels Desktop running Windows – it runs really well when using the Intel platform because that's native for Windows.
Ralph Wessel BArch
Software Engineer Speckle Systems