The eco-nazis have made upgrading coal plants impossible ... If you have an existing coal plant putting out 10 units of emissions a year to produce 1x megawatts and you want to upgrade any part of the plant and make 1.5x megawatts you now have to upgrade the entire plant to new standards of 4 units of emissions which cannot be met.
Conversion to natural gas should not run into these bottlenecks. Nat. gas should meet or beat any possible emissions regime, perhaps only excluding CO2, for which it will be "the" best solution aside from nuclear (i.e. natural gas emits less CO2 per unit of electricity than any other source).