So I had success with a technique I found on another website with an o3DS running 10.3. The instructions were as follows:
- Set your Wi-Fi connections DNS to 107.211.140.065 and exit Settings.
- Turn Wi-Fi off.
- Access the Browser and go to the Bookmarks section and edit an existing bookmark with this website: http://yls8.mtheall.com/3dsbrowserhax_auto.php
- Save bookmark and close browser with Home button. (if you don't the bookmark won't save).
- Turn Wi-Fi on.
- Open the browser and try to access a website. Once the dialog comes up denying access let the browser shut down.
- Edit Wi-Fi connections DNS back to Auto and exit Settings.
- As fast as possible, access the Browser and use the bookmark you made.
- Hopefully success!
This worked for me on an original old 3DS running 10.3 that had the update prompt. I installed MenuHax first thing and proceeded to downgrade it.