Probably a better example from the opposite end of the spectrum would be the design changes between the F/A-18C/D and the F/A-18E/F. The initial plans were to s

Probably a better example from the opposite end of the spectrum would be the design changes between the F/A-18C/D and the F/A-18E/F. The initial plans were to stretch the C/D model to add fuel and improve the abysmal range, but this increased weight which required larger wings which required all sorts of major design changes. In the end, the E/F ended up with only 40% parts commonality, and was now a medium weigh fighter like the F-15, rather than the cheap light fighter like the F-16 it began life as. Engineers I know who worked on the E/F design have told me it would arguably have been cheaper to design a new plane from scratch, but the Navy was so afraid of another failed project that they were only able to sell it to the Navy as an "upgrade" of a plane design they already had. Dubious in an engineering sense, but absolute genius in marketing.