I dont think you can save it back to a "By Default" drawing
But you can save it back to v 2017 by File/ Save As
Then you can open a New drawing from scratch and copy from the v 2017 drawing to the new one by ctrl a, c and v in the new one.
Or use the File/ "Subtract From" tool if you have that, or Tile the windows with both drawings open and just drag the objects over to the New, in case remember to be in Wireframe in both drawings when do so.
V18, V19, V20, V21, 2015/ 16/ 17/ 18 Pro. Platinum
Deluxe V20, V21, 2015/ 16/ 17
Windows 10 Home Premium 64 bit, 32 GB
NVIDIA Geforce GTX 780m, 1 GB