In a plant upgrade EVERY part of the plant would have to be compliant ,, not just meet standards “on average”.
A change from coal to natural gas automatically "upgrades" the whole plant into compliance, because all the "polluting" emissions are eliminated up front. No mercury, no arsenic, no radiation, no high sulfur. The only remaining "emissions" are water vapor, carbon dioxide, and a very small amount of oxides of nitrogen. What else to you think needs to be "upgraded"??? The boiler?? The steam turbine??