Converting Coal Power Plants to Nuclear

I guess this is something that sounds so obvious, why wouldn't you but the devil will be in the details.

As for using the smaller "modular" reactors to take account of the fact that some coal plant infrastructure would be much too small for a large nuclear reactor, I wonder where the sweet spot is between the low up-front investment of a smaller reactor and the potential economy of scale with a larger plant needing fewer workers per MW output.

It makes sense to do this, because you can reuse so much of the infrastructure, the area for the coal pile can be utilized for more power transformers, etc. The main limitations would be grid ties, cooling water, and security.