Natural gas is not being switched out for coal. At least not in the USA. I think that might be a typo. I unfortunately can not site a fancy link like you have but from experience I can tell you that the actual costs I have encountered for projects showed that the price of scrubber tech/equipment (and installation) was less then the cost of a plant conversion from coal to natural gas.
The costs of coal are higher than they were before and are growing. This is a direct result of government regulation. The other problem is that these regulations and the continual call for more is causing a great deal of market uncertainty, which is also driving up costs (not just for the coal but for equipment). Natural gas is currently not in the same position, it is cheaper and less regulated (no mining regulations/legislation). Further, because there has been so much drilled for there is a surplus (supply and demand ==> gas costs are lower). This is mostly because of the lack of good, viable, stable export terminals. So when some terminals open in the future we will see higher natural gas prices.
Coal is being switched out for natural gas here in the US... as a specific example, in 2012, see Plant McDonough right next to Atlanta, GA [1]. As I stated before, the reason is in part due to the effects on air quality (an externality), not just price. For Atlanta with this switch over, it has proven air quality improvements (and no one was surprised by this since it was a coal plant operating full blast right there next to Atlanta!).
I actually agree with you on several points. Agreed that the economics are uncertain, and agreed that scrubbing is very useful, and is definitely worth the cost vs. a full switch over to natural gas (but that doesn't ignore the fact that a switch to natural gas happens for other reasons). And I agree with you that natural gas has higher variance (historically at the least), and higher future expected prices, particularly with exports.
I replied the way I did because you wrote "my guess is that natural gas is being switched out for coal for many reasons". That is not true. That is why I said it might have been a typo. This is clear because in your reply above your write the opposite, that coal is being replaced by natural gas. I agree with this, and as you wrote above, 100%.
The costs of coal are higher than they were before and are growing. This is a direct result of government regulation. The other problem is that these regulations and the continual call for more is causing a great deal of market uncertainty, which is also driving up costs (not just for the coal but for equipment). Natural gas is currently not in the same position, it is cheaper and less regulated (no mining regulations/legislation). Further, because there has been so much drilled for there is a surplus (supply and demand ==> gas costs are lower). This is mostly because of the lack of good, viable, stable export terminals. So when some terminals open in the future we will see higher natural gas prices.