The more I read about legacy (and actively maintained) project refactoring, the more firmly I find myself agreeing that the gradual replacement is the right way

The more I read about legacy (and actively maintained) project refactoring, the more firmly I find myself agreeing that the gradual replacement is the right way to go about things in nearly every case. Whether we like it or not, the old system is a source of truth about how things are done, so the only way to preserve this knowledge fully is to copy the whole thing as-is and then "restate" parts of that knowledge in a more organised/modern way by refactoring, leaving the rest in place.