AutoCAD 2013 :: Changing Units Doesn't Save

I've noticed an anomoly when working with XREF's and changing units.  With the company that I work, we receive client drawings, XREF them in, and put our drawings on top of them.  For this particular project, we have about 150 drawings that we are working with.  The biggest problem is the drawings we receive are in various units, and we draw in millimeters.  So when we go to XATTACH, we have to make sure the XREF drawing's units are set to mm, although most of them are not.  So we navigate to the XREF drawing, open it, open the UNITS window, change it to millimeters, accept, save, and close the drawing.  However, when we go back into the actual drawing to XATTACH again, the unit change in the XREF has not taken effect.  Reopening the drawing and launching UNITS will show that, indeed, the unit change was not saved.  Doing it a second time will take effect, but why do we have to do this dance in the first place?

This problem occurs about 50% of the time.  I can't seem to find a pattern with it.  If I make a change to the XREF drawing itself as well as changing the units (like moving the pan or zoom), but to no avail.

Windows 7 Pro SP1
AutoCAD LT 2013 Version: G.55.0.0