You bet.
Hold volume down + power while powering on.
Look at the top of the screen.
If it says UNLOCKED then get the Mini-SDK -
http://androidforums.com/threads/adb-guide-updated-2014-11-11.443072/#post-5389081
Unzip the sdk-tools.zip file with Windows Explorer, shift+right_click the sdk-tools folder and choose Open command window here.
If you haven't already installed HTC Sync, do that -
http://www.htc.com/us/software/htc-sync-manager/
You can keep it or uninstall it, it'll give you the usb drivers you need for the next part.
Using the onscreen instructions, navigate to FASTBOOT if it's not already there and plug in to your pc - the screen will change to FASTBOOT USB.
In the command window say -
fastboot oem lock
Make sure that you have a 50% charge and follow the manual install instructions here, the download is at the end of the link -
http://www.htc.com/us/support/htc-desire-610-att/news/
Just double click that exe from Windows Explorer, ok that your phone is in FASTBOOT USB mode.
You'll be returned to 100% stock and the phone will receive any future updates from AT&T, just like any other stock phone.
The procedure is going to ERASE your phone first.
Be sure to back up everything that you want to save. Helium Backup is a good choice.