[AnthonyMouse]

They're almost all program and language dependent. If your game doesn't use threads, you don't have to port anything to do with threads. If it does, but you use Java or C++11 and their platform independent thread libraries, you (ideally) don't have to port anything to do with threads. But maybe you used C or C++98 and now you have to run around putting in ifdefs for CreateThread vs. pthread_create and debugging subtle differences between Win32 and POSIX threads, etc.

Or maybe you decided to write the whole thing using multi-platform libraries like Qt in the first place, so that you don't have to change hardly anything. Or maybe you wrote it in Visual Studio using .NET heavily and you have to practically rewrite everything from scratch. It's totally dependent on the decisions you made when writing the original program.