[goto11]

If the code is reasonable well designed (low coupling, separation of concerns and so on) then architectural changes can be introduced by refactoring. I can't imagine an architectural change which will require every single line of code to be rewritten in a realistic application. Most likely there will be large chunks of complex business logic which is largely unaffected by the architectural change.

But migrating away from an obsolete platform probably do requires a massive rewrite. Even then it might be possible to port (rather than rewrite) large chunks.