EPA proposed the rule in December, the then-acting administrator signed it in August and it was made final in October. It was due to have gone into effect this week.
So it looks like this was done under the guise of a rule change.
Most changes are, and they are happening constantly. The CAA doesn't precisely describe certain aspects so it is up to the implementing agency (the EPA in this case) to precisely describe within the intent provided by Congress.
The rules at issue here publicly revolve around the precise definition of maintenance. Could be monetary based, capacity based, etc., but it is defining what is routine maintenance.
Crossing the line that defines routine maintenance means a unit has to install control equipment. Basically, a plant would have been previously permitted to run a process (burn coal in a boiler) and emit pollution up to a point.
The plant decides to do some work on the boiler to make it more efficient. If this work goes past the definition of routine maintenance then they would have to install the controls no matter if the pollution emitted actually decreased with the maintenence. So the change could result in less pollution but the plant would still have to install additional controls to reduce emissions even more.
Basically it is a dis-incentive to make moderate changes that could have positive environmental results to avoid the possibility of having to spend more on additional controls that you aren't required to have if you keep using the same (less efficient) process.
That is balanced by the potential of some plants continuing to fix really poor (in this case high emitting) equipment just to keep it running when it probably should be scrapped for multiple reasons.