I tried this for myself to see if it can be changed back, so I made a small guide for it. Hope it helps.
Whenever I log into YouTube I always ignore the message asking me if I wanted to use my real name so that I can keep my original username. This time I logged out and logged back in and put "Derpy Hooves" as my name.
At the main screen click on the drop down box and click settings.
On the settings page there should be an option called "Disconnect channel from Google+" (It can always be reconnected again later) Click that.
Make sure the checkbox is checked and then click "Rename channel."
After this, your channel should now be reverted back to the original username. :)