I think that they did the cabless units because it was cheaper. All they needed was some sheet metal, in order to make a new hood. Look at SW1000 442. They just put a diamond plate floor where the cab floor was. I think that they just put railings around it.
If they built it back, they would have had to fabricate a new one. The other option would have been to buy a new one from EMD. That cab then needs all fixtures installed.
I don't know if it is true, but I heard that the leasing company was mad about the SD40-2. This unit went to SOO. The owner supposedly said that the unit should have been returned in its original configuration, minus wear and tear.
There was another SD40-2, although not converted to a B unit, had a SD45 hood. I was told that this was due to a wreck too.
When I was a hostler at Clyde Roundhouse, It was a pain to make up some consists. At that time we still had the B30-7 cabless units. Some of the foreign units were a problem too. The foreign units sometimes did not have cab appliances required by CB&Q labor contracts. The result was extra switching moves. Sometimes the only good leader had to be turned.