Several 747-200Bs were sent back to Everett to have the -300's stretched upper-deck fitted. 10% more seats for 2% more weight.
KLM were a big operator, also JAL and UTA. They remained -200s on the type cert, not -300, and were very confusing for spotters...
This was an operation too far, it was found to be too expensive for the gain in accommodation. It also (like the proper -300) gave an improvement in economy, due to different aerodynamic characteristics of the relationship between the end of the upper deck and the wing root. This had first been identified in the 747SP, which had the same positional relationship between these two.
However the rebuilding, known as the 747-200SUD (stretched upper deck) just cost too much to do. The repositioning of the flight controls coming back from the flight deck was a major part of the issue. I believe Boeing eventually lost a lot of money on the programme, so they didn't offer it any more.
I seem to recall being told that Douglas took two damaged DC-9/MD-whatever prototypes, which had been damaged such that there was about one complete airframe left between the two, and "frankensteined" them into one functional airframe. (One of the damaged airframes was the one that had the tail come off during a performance landing test)
The resulting airframe was later used as the UHB testbed.
Both the prototype MD-80s were seriously damaged in tests, one had the whole tail come off as described (video on Youtube), the other had a different issue (landing gear failure ?), and when it was being recovered on the runway the lifting crane overbalanced and fell across the forward fuselage, cutting it in two.
Not a particularly lucky development programme !!!