As demonstrated with the AutoCAD for Mac, this ends up being well behind a single release. Then the program needs to be built back up using those building blocks applicable for the desired operating system - assuming those building blocks exist in the first place. There's a great deal of the program dependent on programmatic building blocks only found under Windows which needs to be stripped out first to get down to a common base program (that includes the user interface as well).
Converting a Windows program to run on a Mac OS is quite complex, and not simply a matter of 'flipping a few switches'.
