The first week of March, Google gave publishers the ability to view their reports in Euros and get paid in either dollars or Euros. Since then, I have noticed many threads from confused publishers asking why are their reports now in Euros. Like it happened by itself.

AdSense Pro Jen chimed in on one of those thread at Google AdSense Help explaining what is going on. She said:

So it seems like currently, you cannot go back to viewing reports in USD once you opt in to seeing them in Euros.

Forum discussion at Google AdSense Help.