If you use Windows, Open Windows Explorer.  Right click on file, choose "Rename", delete ".bak", replace with ".dwg".  Double click on new file, it will open with AutoCAD.

I'm certain there is a similar way to rename files in Apple, but I don't know that.

Engineering is the practice of the art of science - Steve

Tried that but am not able to change the file type.  The filename does not have a .bak extension but that is the type of file it is listed as when I go to details in Explorer.

You can add a period and dwg letters to any file.

Wheen I try to open the renamed file with a .dwg extension I get an error from CAD that say cant find the specified file verify that the file exists.  When I try to open the file from CAD I cannot find it because it it not recognizing the file as a .dwg

When you go into Windows Explorer to rename the file do you know if you have the "Hide extensions for know file types checked" if this is checked then you have renamed the file from xxx.bak to xxx.dwg.bak and so ACAD will not see it as a .DWG file.
 
If you go into Windows Explorer then on the TOOLS menu, then the VIEW tab of the FOLDER OPTIONS menu you can uncheck "Hide extensions for known files" then you should be able to rename the file.

That was the problem.  Thank you very much.