Looking at information around the 747-400 and 747-400D it looks as if airframes could be converted between the types. As per this thread: https://www.airliners.net/forum/viewtopic.php?t=772803 (https://www.airliners.net/forum/viewtopic.php?t=772803) I think it could help find more uses (such as freight conversion) for the 747-400D especially.