First of all, let’s get this straight: millimeter or meter is meaningless for a single drawing, for example, the drawing unit is set to millimeter, and you draw a line of 1000 units long, then you change the drawing units to meter, but the line is still 1000 units long.

The kind of unit you set in the Drawing Units dialog box is inserted units, which controls the unit of measurement for blocks and drawings that are inserted into the current drawing.

If we want to turn a line of 1000 units to 1, we’ll need to use SC (SCALE) command to scale the objects and set the scale factor to 0.001.
If we want to change the dimension of a line of 1000 units to 1, just adjust the Measurement scale and set it to 0.001, shown as below,