The conversion consists of the structural modifications necessary for the installation of a main cabin cargo door. Along with the cargo door, many other structural modifications are accomplished to enhance the service life of the aircraft and increase its gross weight capability. For example, reinforcements are made to the fuselage and wings, and in many cases, the existing cabin floor structure is replaced with higher strength components to increase the load capacity of the cabin floor.
Many systems changes are made, mainly around the conversion of the flight deck from a three-crew setup to a two person crew just like the MD11. This requires extensive avionics and other modifications.
The aircraft also gets a complete structural and systems inspection and repairs (like a complete overhaul) to make sure the aircraft is in top-notch shape and will last a long time in service (at least long enough to make the modification costs worth it). The engines, for example are replaced with overhauled ones of the same type.
A majority of the wire harnesses are replaced, as they existing ones are rather old, and deterioration of insulation would be a concern if the aircraft were continued in service for another 20 years.
The project itself requires several months to accomplish, and requires over 100,000 man-hours of labor.
Hope this gives you some idea of what is involved.