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:
I've been in contact with the Dublin office on this concern and have the following clarification for publishers seeing this announcement:
Before you opt into this feature: - AdSense Reports will be shown in USD - You can choose to get paid in USD or Euro
After you opt into this feature: - AdSense Reports will be shown in Euro - You can still choose to get paid in USD or Euro. Your payment currency is entirely up to you, and does not change after the T&C's are accepted to have Euro as the reporting currency. In other words, whatever payment method and payment currency you had before you accept the new T&C's will remain unchanged after you accept the T&C's.We apologize for any confusion that the announcement is causing and for any offense that may have been taken by the wording.
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.