In Didger 4 and 5, the best way to change the Display Units are to do so in the Coordinate Manager. If you look in the Coordinate Manager, there is a drop down menu at the bottom 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 October 6, 2016