Green Energy

Highly optimistic. For one, the specific parameters of the core dictate the shape, size and configuration of all of the apparatus around it. You can't just slot in a different heat generator and call it good to go. Secondly, there's no guarantee that your future fusion reactor core even fits in the footprint of your fission plant. You have no idea what size and shape it will take.

Finally, your assertion is incorrect. Steam turbines, heat exchangers, and cooling towers are comparatively simple, low risk, and well understood parts of a nuclear reactor. The safety features, checks, design reviews, bureaucracy and permitting surrounding the core itself are what take the most time. Any part that could lead to radioactive containment breach.