Can you delete a working unit from a dgn file?

I have a Microstion file that I am assuming has some sort of corruption as the FOOT working unit is not available. The file is from one of our other offices. Our office took over the project so exactly where all of the files came from and how they were developed is a mystery since all the staff from the other office have departed the company. I do have a copy of this file that has the correct units set & available in the list. We only discovered this problem today as I referenced this file to the main roadway design file and they did not line up. Through the investigation I found the working unit was apparently changed to use Survey Feet rather than International Feet. I went into the Working Units dialog to switch the working units and found that the unit of Feet was missing. There is another model contained in this file and switching to that model it is set to Feet. This leads me to believe the file is corrupted, but I want to know if you are able to actually delete a unit from a file even if it is defined in the units.def file. My assumption and common sense tells me this is not possible.

Microstation v8i