Sometimes it works. Like how the A-7 Corsair II was a "derivative" of the F-8 Crusader.
Not the same thing at all, the Super Bug is one of the best examples, it's
a new aircraft yet still designated as an F-18.
That's the game, make major changes yet claim it's still the same basic
aircraft. It's been done in the commercial aviation arena for yonks, it's a
way around having to go through the new design certification process.
The current 747-8 has very little left in common with the original 747.