Please note that QCad does not scale your drawing for you when changing the drawing unit.
For example if a line was 10cm long and you changed the drawing unit to Millimeter, the line is now 10mm long. In fact, the drawing unit does not have any influence on the drawing geometry. It is only used when printing to scale.
If you want to convert your drawing from cm to mm, please use the scale function in addition to changing the unit in the drawing preferences.