Here is some math I came up with using the the US EPA.
28% of GHG is electricity
67% of electricity GHG is coal
Therefore 19% of GHG in US is coal
Therefore switching coal to natural gas could reduce roughly 10% of total US GHG.
28% of GHG is transportation
55% of transportation GHG is private cars and small trucks used for passenger transport
Therefore 15% of GHG is private cars and trucks.
So in total, to put it in magnitude, switching US coal to natural gas is the same as removing two-thirds of the cars and car-based travel of the entire United States.
In terms of feasibility, GE could probably crunch out the turbines by the end of next year. Certainly within five years. Five years is about when the request for proposals, for a new nuclear power plant, gets first reading.