I wanted to mention this somewhere since I haven't seen this in any forum and thought it might help someone else.
We recently faced this issue in our work group where one of our members did not have access to AutoCAD for quick rescaling of a part they were working on. This is what we discovered in the way Fusion creates dxfs.
Fusion uses the drawing standard, ASME or ISO, as the indicator for the type of dxf it creates.
ASME = inch scaling
ISO = mm scaling
The standard is only selectable on drawing creation. So if you need the opposing scaling you have to create a new drawing in the correct standard.
To check what standard a drawing is using go into the drawing and open the "Document Settings" in the navigation browser.