System development (and game development) doesn't start with coding, but with discussion to define process flows, 'business/game' rules and logic structures.
Coding is converting these into computer algorithms.
Hence I think you'll find a lot of thought and time would have already gone into the Soldier AI and Unit AI, how it relates to and interacts with the 'upper echelon' AIs, and how it all works together.
Also remember the upper echelon AIs have already been developed (and already appear in the current CM engine) and appear to do the job quite well, and I would expect them to use those as a basis for the new engine, with maybe some tweaks and fine tuning.
As for OS conversion, I don't believe this is as big an issue as the core coding will be the same regardless of Operating System, with conversion more addressing how each OS handles graphics, interface, file management etc.
Another influencing factor is programmer experience.
To get a new coder will not necessarily speed things up at first because the coder has to review and learn the existing code and know how it hangs together...there will always be a learning curve that will affect that person's output.
Mace