The best way to change the Display Units are to do so in the Coordinate Manager. At the bottom, there is a drop down menu for Display Units. Changing them here will just change the units and not do the reprojection.
You can also change the display units under Map | Change Projection, but if you are not changing the projection or datum as well, this is not ideal as it will still perform the projection transformation calculation and can take a while (even if you didn't change anything else besides the Display Units).
Updated May 1, 2018