If your DEM is in meters, I think you will have to recalculate the values in the raster. Not such a difficult thing, with Raster Calculator just divide the whole thing by 3.28 to create a new raster layer in feet. At least, that's what I would do. You can also change the basic measurement units of your GIS project at View -> Data Frame Properties -> General, but I am pretty sure it will not change the pixel values of your DEM for you.
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