What would it mean for all current OSX software?
Apple would likely rewrite all of their official apps to work on ARM, but what about the thousands of programs running on Intel based OSX machines?
What is this, 1968? Apple likely would have to rewrite nothing as they already have the kernel running on ARM (iOS) and their software is written in C/C++/Objective C, which is highly portable and very efficiently compiled due to the LLVM project which they are active contributors to.