there is no such thing. Complete rewrites don't exist. even the jump from OS9 to OSX it wasn't a complete rewrite.old code was reused where appropriate. Noone rewrites from scratch, technically it's not even possibly, even if you where to start from complete scratcha nd not even reference old code, it'd be in your memory or you'd have learned about it and you'd use it.
I agree with you. Not sure the step specifically from OS 9 to OS X is a good example though, because most of OS 9's code actually did
get left behind, simply because OS X architecturally continued NeXTStep's legacy much more than it did OS 9's.