You and @sjrcpawere both correct. I went in and simply made changes to the original. The D-40 (DC Amended Individual Tax Return) has a box at the top of the form indicating the return is an amendment. This box was marked with an X after the change was made. It did not create a D-40X, but I guess they are no longer using that form - as per DC's List of Individual Income Tax Forms for the 2019 Tax Year.
Thank you both.