The command Calibrate does dynamic changes of the Unit System of the drawing.
There are three modes:
1) Change Unit and apply a scale factor:
2) Select two points as reference and set the new reference distance:
3) Select a polygon and apply the new reference distance:
Note: For snap-modes to work, the drawings must be converted using the conversion parameter hlall, see documentation for AutoCAD Handle-Based Processing, so in the conversion there is an associated handle for the object to snap to.
Also see Settings on how to dynamically add a parameter.