Google's premium ad network for professional publishers is AdX which is run via the google ad manager product and has been on a CPM basis for over a decade. Only small sites are on adsense which pays CPC.

The net change here is probably almost nothing, just the smaller sites that never use google ad manager will see the change but any publisher of note will have been operating with this for as long as they can remember.

With click through rates continuing to decrease it's likely they needed to make this change to keep the long tail sites happy and generating some revenue, they would back out the CPC to an effective CPM anyway.

Me:13+ years in digital publishing and advertising.