Nema 12 includes gasketing on the front, back, top, bottom and sides of the MCC.  An existing Model 6 MCC cannot be converted from Nema 1 to Nema 12 in the field because the factory includes gasketing between the sections.  Nema 12 also includes bottom plates that are attached from the bottom of the section.  Also, if there are drives in the MCC, the Nema 12 drives may require larger units due to the ventilation requirements to meet Nema 12.