Well, regardless of unit measurement all objects are drawn in 'units' of 1 (decimal parts thereof), 1 unit may equal 1 inch, 1 mile or 1 millimetre but they always equal one, you set the drg unit's to be what you want them to be.
So given that, if you want a drawing converted from inches to mm you would need to scale the drawing by 25.4 to turn 1 unit (inch) into 25.4 units (25.4mm). Reverse for the other way, you would also need to tell acad what unit measurements you are now using.
hope that's a start for you.